diff --git a/ApiCore-Frontend/src/app/(app)/feedings/new/page.tsx b/ApiCore-Frontend/src/app/(app)/feedings/new/page.tsx
index 1a10c49..3e9ac6a 100644
--- a/ApiCore-Frontend/src/app/(app)/feedings/new/page.tsx
+++ b/ApiCore-Frontend/src/app/(app)/feedings/new/page.tsx
@@ -11,6 +11,7 @@ import { FeedType } from "@/lib/feedings/types";
 import { FEED_TYPES, FEED_TYPE_LABELS } from "@/lib/feedings/labels";
 import { ApiError } from "@/lib/auth/api-client";
 import { queueFeeding } from "@/lib/offline/db";
+import { newIdempotencyKey } from "@/lib/offline/idempotency";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
@@ -50,8 +51,12 @@ export default function NewFeedingPage() {
   const [notes, setNotes] = useState("");
   const [error, setError] = useState<string | null>(null);
 
+  // One key per opened form: a double tap or an offline retry of the
+  // same entry is recognised by the server instead of saved twice.
+  const [idempotencyKey] = useState(newIdempotencyKey);
   const mutation = useMutation({
-    mutationFn: createFeeding,
+    mutationFn: (input: Parameters<typeof createFeeding>[0]) =>
+      createFeeding(input, { idempotencyKey }),
     onSuccess: (feeding) => {
       void queryClient.invalidateQueries({
         queryKey: ["colonies", feeding.colonyId, "feedings"],
@@ -67,7 +72,7 @@ export default function NewFeedingPage() {
         setError("Fütterung konnte nicht gespeichert werden.");
         return;
       }
-      void queueFeeding(variables).then(() => {
+      void queueFeeding(variables, idempotencyKey).then(() => {
         toast.info("Kein Netzwerk – Fütterung wurde offline gespeichert und wird synchronisiert.");
         router.push(`/colonies/${variables.colonyId}`);
       });
diff --git a/ApiCore-Frontend/src/app/(app)/harvests/new/page.tsx b/ApiCore-Frontend/src/app/(app)/harvests/new/page.tsx
index 76aa604..32a2a22 100644
--- a/ApiCore-Frontend/src/app/(app)/harvests/new/page.tsx
+++ b/ApiCore-Frontend/src/app/(app)/harvests/new/page.tsx
@@ -11,6 +11,7 @@ import { createHarvest } from "@/lib/harvests/api";
 import { fetchWithdrawalStatus } from "@/lib/treatments/api";
 import { ApiError } from "@/lib/auth/api-client";
 import { queueHarvest } from "@/lib/offline/db";
+import { newIdempotencyKey } from "@/lib/offline/idempotency";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
@@ -60,8 +61,12 @@ export default function NewHarvestPage() {
     (withdrawalStatuses ?? []).map((status) => [status.colonyId, status]),
   );
 
+  // One key per opened form: a double tap or an offline retry of the
+  // same entry is recognised by the server instead of saved twice.
+  const [idempotencyKey] = useState(newIdempotencyKey);
   const mutation = useMutation({
-    mutationFn: createHarvest,
+    mutationFn: (input: Parameters<typeof createHarvest>[0]) =>
+      createHarvest(input, { idempotencyKey }),
     onSuccess: (harvest) => {
       void queryClient.invalidateQueries({ queryKey: ["harvests"] });
       toast.success("Ernte gespeichert");
@@ -72,7 +77,7 @@ export default function NewHarvestPage() {
         setError("Ernte konnte nicht gespeichert werden.");
         return;
       }
-      void queueHarvest(variables).then(() => {
+      void queueHarvest(variables, idempotencyKey).then(() => {
         toast.info("Kein Netzwerk – Ernte wurde offline gespeichert und wird synchronisiert.");
         router.push("/harvests");
       });
diff --git a/ApiCore-Frontend/src/app/(app)/inspections/new/page.tsx b/ApiCore-Frontend/src/app/(app)/inspections/new/page.tsx
index 5b422c7..5240d34 100644
--- a/ApiCore-Frontend/src/app/(app)/inspections/new/page.tsx
+++ b/ApiCore-Frontend/src/app/(app)/inspections/new/page.tsx
@@ -9,6 +9,7 @@ import { fetchColonies } from "@/lib/colonies/api";
 import { createInspection } from "@/lib/inspections/api";
 import { ApiError } from "@/lib/auth/api-client";
 import { queueInspection } from "@/lib/offline/db";
+import { newIdempotencyKey } from "@/lib/offline/idempotency";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
@@ -69,8 +70,12 @@ export default function NewInspectionPage() {
   const [notes, setNotes] = useState("");
   const [error, setError] = useState<string | null>(null);
 
+  // One key per opened form: a double tap or an offline retry of the
+  // same entry is recognised by the server instead of saved twice.
+  const [idempotencyKey] = useState(newIdempotencyKey);
   const mutation = useMutation({
-    mutationFn: createInspection,
+    mutationFn: (input: Parameters<typeof createInspection>[0]) =>
+      createInspection(input, { idempotencyKey }),
     onSuccess: (inspection) => {
       void queryClient.invalidateQueries({
         queryKey: ["colonies", inspection.colonyId, "inspections"],
@@ -83,7 +88,7 @@ export default function NewInspectionPage() {
         setError("Durchsicht konnte nicht gespeichert werden.");
         return;
       }
-      void queueInspection(variables).then(() => {
+      void queueInspection(variables, idempotencyKey).then(() => {
         toast.info("Kein Netzwerk – Durchsicht wurde offline gespeichert und wird synchronisiert.");
         router.push(`/colonies/${variables.colonyId}`);
       });
diff --git a/ApiCore-Frontend/src/app/(app)/swarm-control/new/page.tsx b/ApiCore-Frontend/src/app/(app)/swarm-control/new/page.tsx
index 18c64b1..9540486 100644
--- a/ApiCore-Frontend/src/app/(app)/swarm-control/new/page.tsx
+++ b/ApiCore-Frontend/src/app/(app)/swarm-control/new/page.tsx
@@ -11,6 +11,7 @@ import { SWARM_ACTION_LABELS, SWARM_MOOD_LABELS } from "@/lib/swarm-control/labe
 import { SwarmControlAction, SwarmMood } from "@/lib/swarm-control/types";
 import { ApiError } from "@/lib/auth/api-client";
 import { queueSwarmControlCheck } from "@/lib/offline/db";
+import { newIdempotencyKey } from "@/lib/offline/idempotency";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
@@ -50,8 +51,12 @@ export default function NewSwarmControlCheckPage() {
   const [notes, setNotes] = useState("");
   const [error, setError] = useState<string | null>(null);
 
+  // One key per opened form: a double tap or an offline retry of the
+  // same entry is recognised by the server instead of saved twice.
+  const [idempotencyKey] = useState(newIdempotencyKey);
   const mutation = useMutation({
-    mutationFn: createSwarmControlCheck,
+    mutationFn: (input: Parameters<typeof createSwarmControlCheck>[0]) =>
+      createSwarmControlCheck(input, { idempotencyKey }),
     onSuccess: (check) => {
       void queryClient.invalidateQueries({
         queryKey: ["colonies", check.colonyId, "swarm-control-checks"],
@@ -64,7 +69,7 @@ export default function NewSwarmControlCheckPage() {
         setError("Schwarmkontrolle konnte nicht gespeichert werden.");
         return;
       }
-      void queueSwarmControlCheck(variables).then(() => {
+      void queueSwarmControlCheck(variables, idempotencyKey).then(() => {
         toast.info("Kein Netzwerk – Schwarmkontrolle wurde offline gespeichert und wird synchronisiert.");
         router.push(`/colonies/${variables.colonyId}`);
       });
diff --git a/ApiCore-Frontend/src/app/(app)/treatments/new/page.tsx b/ApiCore-Frontend/src/app/(app)/treatments/new/page.tsx
index 0c2cb56..9169825 100644
--- a/ApiCore-Frontend/src/app/(app)/treatments/new/page.tsx
+++ b/ApiCore-Frontend/src/app/(app)/treatments/new/page.tsx
@@ -9,6 +9,7 @@ import { fetchColonies } from "@/lib/colonies/api";
 import { createTreatment } from "@/lib/treatments/api";
 import { ApiError } from "@/lib/auth/api-client";
 import { queueTreatment } from "@/lib/offline/db";
+import { newIdempotencyKey } from "@/lib/offline/idempotency";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
@@ -52,8 +53,12 @@ export default function NewTreatmentPage() {
   const [withdrawalPeriodDays, setWithdrawalPeriodDays] = useState("");
   const [error, setError] = useState<string | null>(null);
 
+  // One key per opened form: a double tap or an offline retry of the
+  // same entry is recognised by the server instead of saved twice.
+  const [idempotencyKey] = useState(newIdempotencyKey);
   const mutation = useMutation({
-    mutationFn: createTreatment,
+    mutationFn: (input: Parameters<typeof createTreatment>[0]) =>
+      createTreatment(input, { idempotencyKey }),
     onSuccess: (treatment) => {
       void queryClient.invalidateQueries({
         queryKey: ["colonies", treatment.colonyId, "treatments"],
@@ -66,7 +71,7 @@ export default function NewTreatmentPage() {
         setError("Behandlung konnte nicht gespeichert werden.");
         return;
       }
-      void queueTreatment(variables).then(() => {
+      void queueTreatment(variables, idempotencyKey).then(() => {
         toast.info("Kein Netzwerk – Behandlung wurde offline gespeichert und wird synchronisiert.");
         router.push(`/colonies/${variables.colonyId}`);
       });
diff --git a/ApiCore-Frontend/src/app/(app)/varroa/new/page.tsx b/ApiCore-Frontend/src/app/(app)/varroa/new/page.tsx
index d124442..1fe5f23 100644
--- a/ApiCore-Frontend/src/app/(app)/varroa/new/page.tsx
+++ b/ApiCore-Frontend/src/app/(app)/varroa/new/page.tsx
@@ -11,6 +11,7 @@ import { VarroaMethod } from "@/lib/varroa/types";
 import { VARROA_METHODS, VARROA_METHOD_LABELS } from "@/lib/varroa/labels";
 import { ApiError } from "@/lib/auth/api-client";
 import { queueVarroaMeasurement } from "@/lib/offline/db";
+import { newIdempotencyKey } from "@/lib/offline/idempotency";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
@@ -49,8 +50,12 @@ export default function NewVarroaMeasurementPage() {
   const [notes, setNotes] = useState("");
   const [error, setError] = useState<string | null>(null);
 
+  // One key per opened form: a double tap or an offline retry of the
+  // same entry is recognised by the server instead of saved twice.
+  const [idempotencyKey] = useState(newIdempotencyKey);
   const mutation = useMutation({
-    mutationFn: createVarroaMeasurement,
+    mutationFn: (input: Parameters<typeof createVarroaMeasurement>[0]) =>
+      createVarroaMeasurement(input, { idempotencyKey }),
     onSuccess: (measurement) => {
       void queryClient.invalidateQueries({
         queryKey: ["colonies", measurement.colonyId, "varroa-measurements"],
@@ -63,7 +68,7 @@ export default function NewVarroaMeasurementPage() {
         setError("Varroamessung konnte nicht gespeichert werden.");
         return;
       }
-      void queueVarroaMeasurement(variables).then(() => {
+      void queueVarroaMeasurement(variables, idempotencyKey).then(() => {
         toast.info("Kein Netzwerk – Varroamessung wurde offline gespeichert und wird synchronisiert.");
         router.push(`/colonies/${variables.colonyId}`);
       });
diff --git a/ApiCore-Frontend/src/lib/feedings/api.ts b/ApiCore-Frontend/src/lib/feedings/api.ts
index 70d4333..1e7b217 100644
--- a/ApiCore-Frontend/src/lib/feedings/api.ts
+++ b/ApiCore-Frontend/src/lib/feedings/api.ts
@@ -1,4 +1,5 @@
 import { apiFetch } from "@/lib/auth/api-client";
+import { CreateOptions, idempotencyHeaders } from "@/lib/offline/idempotency";
 import {
   CreateFeedingInput,
   Feeding,
@@ -14,9 +15,13 @@ export function fetchColonyFeedingsSummary(colonyId: string): Promise<FeedingSum
   return apiFetch<FeedingSummaryEntry[]>(`/colonies/${colonyId}/feedings/summary`);
 }
 
-export function createFeeding(input: CreateFeedingInput): Promise<Feeding> {
+export function createFeeding(
+  input: CreateFeedingInput,
+  options: CreateOptions = {},
+): Promise<Feeding> {
   return apiFetch<Feeding>("/feedings", {
     method: "POST",
+    headers: idempotencyHeaders(options),
     body: JSON.stringify(input),
   });
 }
diff --git a/ApiCore-Frontend/src/lib/harvests/api.ts b/ApiCore-Frontend/src/lib/harvests/api.ts
index 6428fbe..723e59b 100644
--- a/ApiCore-Frontend/src/lib/harvests/api.ts
+++ b/ApiCore-Frontend/src/lib/harvests/api.ts
@@ -1,4 +1,5 @@
 import { apiFetch } from "@/lib/auth/api-client";
+import { CreateOptions, idempotencyHeaders } from "@/lib/offline/idempotency";
 import { CreateHarvestInput, Harvest, UpdateHarvestInput } from "./types";
 
 export function fetchHarvests(): Promise<Harvest[]> {
@@ -9,9 +10,13 @@ export function fetchHarvest(id: string): Promise<Harvest> {
   return apiFetch<Harvest>(`/harvests/${id}`);
 }
 
-export function createHarvest(input: CreateHarvestInput): Promise<Harvest> {
+export function createHarvest(
+  input: CreateHarvestInput,
+  options: CreateOptions = {},
+): Promise<Harvest> {
   return apiFetch<Harvest>("/harvests", {
     method: "POST",
+    headers: idempotencyHeaders(options),
     body: JSON.stringify(input),
   });
 }
diff --git a/ApiCore-Frontend/src/lib/inspections/api.ts b/ApiCore-Frontend/src/lib/inspections/api.ts
index 898ddd4..3807b71 100644
--- a/ApiCore-Frontend/src/lib/inspections/api.ts
+++ b/ApiCore-Frontend/src/lib/inspections/api.ts
@@ -1,4 +1,5 @@
 import { apiFetch } from "@/lib/auth/api-client";
+import { CreateOptions, idempotencyHeaders } from "@/lib/offline/idempotency";
 import {
   CreateInspectionInput,
   CreateTaskFromInspectionInput,
@@ -11,9 +12,13 @@ export function fetchColonyInspections(colonyId: string): Promise<Inspection[]>
   return apiFetch<Inspection[]>(`/colonies/${colonyId}/inspections`);
 }
 
-export function createInspection(input: CreateInspectionInput): Promise<Inspection> {
+export function createInspection(
+  input: CreateInspectionInput,
+  options: CreateOptions = {},
+): Promise<Inspection> {
   return apiFetch<Inspection>("/inspections", {
     method: "POST",
+    headers: idempotencyHeaders(options),
     body: JSON.stringify(input),
   });
 }
diff --git a/ApiCore-Frontend/src/lib/offline/db.ts b/ApiCore-Frontend/src/lib/offline/db.ts
index 32c4481..1fbb8a3 100644
--- a/ApiCore-Frontend/src/lib/offline/db.ts
+++ b/ApiCore-Frontend/src/lib/offline/db.ts
@@ -18,6 +18,8 @@ interface PendingEntry<T> {
   id: number;
   input: T;
   queuedAt: string;
+  // Missing on entries queued before keys existed - those sync as before.
+  idempotencyKey?: string;
 }
 
 interface OfflineDB extends DBSchema {
@@ -70,9 +72,13 @@ function getDb(): Promise<IDBPDatabase<OfflineDB>> {
   return dbPromise;
 }
 
-async function queue<T>(store: PendingStoreName, input: T): Promise<void> {
+async function queue<T>(
+  store: PendingStoreName,
+  input: T,
+  idempotencyKey?: string,
+): Promise<void> {
   const db = await getDb();
-  await db.add(store, { input, queuedAt: new Date().toISOString() } as never);
+  await db.add(store, { input, idempotencyKey, queuedAt: new Date().toISOString() } as never);
 }
 
 async function getPending<T>(store: PendingStoreName): Promise<PendingEntry<T>[]> {
@@ -91,8 +97,8 @@ async function countPending(store: PendingStoreName): Promise<number> {
 }
 
 export type PendingInspection = PendingEntry<CreateInspectionInput>;
-export function queueInspection(input: CreateInspectionInput): Promise<void> {
-  return queue("pending-inspections", input);
+export function queueInspection(input: CreateInspectionInput, idempotencyKey?: string): Promise<void> {
+  return queue("pending-inspections", input, idempotencyKey);
 }
 export function getPendingInspections(): Promise<PendingInspection[]> {
   return getPending("pending-inspections");
@@ -105,8 +111,8 @@ export function countPendingInspections(): Promise<number> {
 }
 
 export type PendingTreatment = PendingEntry<CreateTreatmentInput>;
-export function queueTreatment(input: CreateTreatmentInput): Promise<void> {
-  return queue("pending-treatments", input);
+export function queueTreatment(input: CreateTreatmentInput, idempotencyKey?: string): Promise<void> {
+  return queue("pending-treatments", input, idempotencyKey);
 }
 export function getPendingTreatments(): Promise<PendingTreatment[]> {
   return getPending("pending-treatments");
@@ -119,8 +125,8 @@ export function countPendingTreatments(): Promise<number> {
 }
 
 export type PendingFeeding = PendingEntry<CreateFeedingInput>;
-export function queueFeeding(input: CreateFeedingInput): Promise<void> {
-  return queue("pending-feedings", input);
+export function queueFeeding(input: CreateFeedingInput, idempotencyKey?: string): Promise<void> {
+  return queue("pending-feedings", input, idempotencyKey);
 }
 export function getPendingFeedings(): Promise<PendingFeeding[]> {
   return getPending("pending-feedings");
@@ -133,8 +139,8 @@ export function countPendingFeedings(): Promise<number> {
 }
 
 export type PendingVarroaMeasurement = PendingEntry<CreateVarroaMeasurementInput>;
-export function queueVarroaMeasurement(input: CreateVarroaMeasurementInput): Promise<void> {
-  return queue("pending-varroa-measurements", input);
+export function queueVarroaMeasurement(input: CreateVarroaMeasurementInput, idempotencyKey?: string): Promise<void> {
+  return queue("pending-varroa-measurements", input, idempotencyKey);
 }
 export function getPendingVarroaMeasurements(): Promise<PendingVarroaMeasurement[]> {
   return getPending("pending-varroa-measurements");
@@ -147,8 +153,8 @@ export function countPendingVarroaMeasurements(): Promise<number> {
 }
 
 export type PendingSwarmControlCheck = PendingEntry<CreateSwarmControlCheckInput>;
-export function queueSwarmControlCheck(input: CreateSwarmControlCheckInput): Promise<void> {
-  return queue("pending-swarm-control-checks", input);
+export function queueSwarmControlCheck(input: CreateSwarmControlCheckInput, idempotencyKey?: string): Promise<void> {
+  return queue("pending-swarm-control-checks", input, idempotencyKey);
 }
 export function getPendingSwarmControlChecks(): Promise<PendingSwarmControlCheck[]> {
   return getPending("pending-swarm-control-checks");
@@ -161,8 +167,8 @@ export function countPendingSwarmControlChecks(): Promise<number> {
 }
 
 export type PendingHarvest = PendingEntry<CreateHarvestInput>;
-export function queueHarvest(input: CreateHarvestInput): Promise<void> {
-  return queue("pending-harvests", input);
+export function queueHarvest(input: CreateHarvestInput, idempotencyKey?: string): Promise<void> {
+  return queue("pending-harvests", input, idempotencyKey);
 }
 export function getPendingHarvests(): Promise<PendingHarvest[]> {
   return getPending("pending-harvests");
diff --git a/ApiCore-Frontend/src/lib/offline/idempotency.ts b/ApiCore-Frontend/src/lib/offline/idempotency.ts
new file mode 100644
index 0000000..f45df8f
--- /dev/null
+++ b/ApiCore-Frontend/src/lib/offline/idempotency.ts
@@ -0,0 +1,23 @@
+/**
+ * One key per form submission, sent as `Idempotency-Key` and stored with an
+ * offline-queued entry. If the server saved a record but the answer never
+ * reached the phone (dead zone at the apiary), the queued retry carries the
+ * same key and the server returns the first record instead of a duplicate.
+ */
+export interface CreateOptions {
+  idempotencyKey?: string;
+}
+
+export function idempotencyHeaders(options: CreateOptions = {}): HeadersInit {
+  return options.idempotencyKey ? { "Idempotency-Key": options.idempotencyKey } : {};
+}
+
+export function newIdempotencyKey(): string {
+  // randomUUID only exists in secure contexts (HTTPS/localhost); the app can
+  // also be opened over plain HTTP in a local network.
+  if (typeof crypto.randomUUID === "function") {
+    return crypto.randomUUID();
+  }
+  const bytes = crypto.getRandomValues(new Uint8Array(16));
+  return Array.from(bytes, (byte) => byte.toString(16).padStart(2, "0")).join("");
+}
diff --git a/ApiCore-Frontend/src/lib/offline/sync.spec.ts b/ApiCore-Frontend/src/lib/offline/sync.spec.ts
index 089d332..eb43bb2 100644
--- a/ApiCore-Frontend/src/lib/offline/sync.spec.ts
+++ b/ApiCore-Frontend/src/lib/offline/sync.spec.ts
@@ -54,6 +54,26 @@ describe("syncPendingInspections", () => {
     expect(mockedRemovePending).toHaveBeenCalledWith(2);
   });
 
+  it("resends a queued entry with the idempotency key it was queued with", async () => {
+    // The server may already have saved the first attempt; the same key lets
+    // it answer with that record instead of storing a duplicate.
+    mockedGetPending.mockResolvedValue([
+      { ...pendingEntry(1), idempotencyKey: "key-from-the-form-0001" },
+      pendingEntry(2),
+    ]);
+    mockedCreateInspection.mockResolvedValue({ id: "x" } as never);
+
+    await syncPendingInspections();
+
+    expect(mockedCreateInspection).toHaveBeenNthCalledWith(1, pendingEntry(1).input, {
+      idempotencyKey: "key-from-the-form-0001",
+    });
+    // Entries queued before keys existed still sync, just without one.
+    expect(mockedCreateInspection).toHaveBeenNthCalledWith(2, pendingEntry(2).input, {
+      idempotencyKey: undefined,
+    });
+  });
+
   it("keeps a rejected entry queued and continues with the rest on a server-side ApiError", async () => {
     mockedGetPending.mockResolvedValue([pendingEntry(1), pendingEntry(2)]);
     mockedCreateInspection
diff --git a/ApiCore-Frontend/src/lib/offline/sync.ts b/ApiCore-Frontend/src/lib/offline/sync.ts
index e384c10..0dbb6d7 100644
--- a/ApiCore-Frontend/src/lib/offline/sync.ts
+++ b/ApiCore-Frontend/src/lib/offline/sync.ts
@@ -1,4 +1,5 @@
 import { ApiError } from "@/lib/auth/api-client";
+import { CreateOptions } from "./idempotency";
 import { createInspection } from "@/lib/inspections/api";
 import { createTreatment } from "@/lib/treatments/api";
 import { createFeeding } from "@/lib/feedings/api";
@@ -46,9 +47,9 @@ export function syncPendingInspections(): Promise<SyncResult> {
 // failed and left in the queue too, rather than silently dropping data the
 // user entered - the sync badge stays visible so it's not forgotten.
 async function drain<T>(
-  getPending: () => Promise<{ id: number; input: T }[]>,
+  getPending: () => Promise<{ id: number; input: T; idempotencyKey?: string }[]>,
   removePending: (id: number) => Promise<void>,
-  create: (input: T) => Promise<unknown>,
+  create: (input: T, options: CreateOptions) => Promise<unknown>,
 ): Promise<SyncResult> {
   const pending = await getPending();
   let synced = 0;
@@ -56,7 +57,7 @@ async function drain<T>(
 
   for (const entry of pending) {
     try {
-      await create(entry.input);
+      await create(entry.input, { idempotencyKey: entry.idempotencyKey });
       await removePending(entry.id);
       synced += 1;
     } catch (error) {
diff --git a/ApiCore-Frontend/src/lib/swarm-control/api.ts b/ApiCore-Frontend/src/lib/swarm-control/api.ts
index 5c44fa1..b5a5916 100644
--- a/ApiCore-Frontend/src/lib/swarm-control/api.ts
+++ b/ApiCore-Frontend/src/lib/swarm-control/api.ts
@@ -1,4 +1,5 @@
 import { apiFetch } from "@/lib/auth/api-client";
+import { CreateOptions, idempotencyHeaders } from "@/lib/offline/idempotency";
 import {
   CreateSwarmControlCheckInput,
   SwarmControlCheck,
@@ -11,9 +12,11 @@ export function fetchColonySwarmControlChecks(colonyId: string): Promise<SwarmCo
 
 export function createSwarmControlCheck(
   input: CreateSwarmControlCheckInput,
+  options: CreateOptions = {},
 ): Promise<SwarmControlCheck> {
   return apiFetch<SwarmControlCheck>("/swarm-control-checks", {
     method: "POST",
+    headers: idempotencyHeaders(options),
     body: JSON.stringify(input),
   });
 }
diff --git a/ApiCore-Frontend/src/lib/treatments/api.ts b/ApiCore-Frontend/src/lib/treatments/api.ts
index 8a55a68..2cc45bb 100644
--- a/ApiCore-Frontend/src/lib/treatments/api.ts
+++ b/ApiCore-Frontend/src/lib/treatments/api.ts
@@ -1,4 +1,5 @@
 import { apiFetch } from "@/lib/auth/api-client";
+import { CreateOptions, idempotencyHeaders } from "@/lib/offline/idempotency";
 import {
   CreateTreatmentInput,
   Treatment,
@@ -21,9 +22,13 @@ export function fetchWithdrawalStatus(
   );
 }
 
-export function createTreatment(input: CreateTreatmentInput): Promise<Treatment> {
+export function createTreatment(
+  input: CreateTreatmentInput,
+  options: CreateOptions = {},
+): Promise<Treatment> {
   return apiFetch<Treatment>("/treatments", {
     method: "POST",
+    headers: idempotencyHeaders(options),
     body: JSON.stringify(input),
   });
 }
diff --git a/ApiCore-Frontend/src/lib/varroa/api.ts b/ApiCore-Frontend/src/lib/varroa/api.ts
index 5a891f1..89c31c1 100644
--- a/ApiCore-Frontend/src/lib/varroa/api.ts
+++ b/ApiCore-Frontend/src/lib/varroa/api.ts
@@ -1,4 +1,5 @@
 import { apiFetch } from "@/lib/auth/api-client";
+import { CreateOptions, idempotencyHeaders } from "@/lib/offline/idempotency";
 import {
   CreateVarroaMeasurementInput,
   UpdateVarroaMeasurementInput,
@@ -11,9 +12,11 @@ export function fetchColonyVarroaMeasurements(colonyId: string): Promise<VarroaM
 
 export function createVarroaMeasurement(
   input: CreateVarroaMeasurementInput,
+  options: CreateOptions = {},
 ): Promise<VarroaMeasurement> {
   return apiFetch<VarroaMeasurement>("/varroa-measurements", {
     method: "POST",
+    headers: idempotencyHeaders(options),
     body: JSON.stringify(input),
   });
 }