diff --git a/src/dedup.ts b/src/dedup.ts
index 485edcd..4357afd 100644
--- a/src/dedup.ts
+++ b/src/dedup.ts
@@ -1,3 +1,5 @@
+import { createHash } from "crypto";
+
 export class Deduplicator<T> {
   private _inflight = new Map<string, Promise<T>>();
   private _hits = 0;
@@ -24,3 +26,38 @@ export class Deduplicator<T> {
     return { deduped: this._hits, total: this._hits + this._misses };
   }
 }
+
+// In-memory key registry for idempotency tracking (#612)
+const _knownKeys = new Set<string>();
+
+/**
+ * Generates a deterministic idempotency key from payment parameters.
+ * The key is a SHA-256 hex digest of `"{invoiceId}:{payer}:{amount}"`
+ * with an optional `:{nonce}` suffix when provided.
+ */
+export function generateIdempotencyKey(params: {
+  invoiceId: string;
+  payer: string;
+  amount: bigint;
+  nonce?: string;
+}): string {
+  const payload = params.nonce
+    ? `${params.invoiceId}:${params.payer}:${params.amount}:${params.nonce}`
+    : `${params.invoiceId}:${params.payer}:${params.amount}`;
+  return createHash("sha256").update(payload).digest("hex");
+}
+
+/** Returns true if the key has already been registered. */
+export function isKnownKey(key: string): boolean {
+  return _knownKeys.has(key);
+}
+
+/** Registers a key as known (idempotent). */
+export function registerKey(key: string): void {
+  _knownKeys.add(key);
+}
+
+/** Clears the in-memory key registry. Intended for test teardown. */
+export function clearKeys(): void {
+  _knownKeys.clear();
+}
diff --git a/src/index.ts b/src/index.ts
index ebf7e30..a68bda2 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -277,7 +277,13 @@ export {
   buildRevealTransactionFromStorage,
 } from "./confidential.js";
 
-export { Deduplicator } from "./dedup.js";
+export {
+  Deduplicator,
+  generateIdempotencyKey,
+  isKnownKey,
+  registerKey,
+  clearKeys,
+} from "./dedup.js";
 
 export { TxQueue } from "./queue.js";
 
diff --git a/test/dedup.test.ts b/test/dedup.test.ts
new file mode 100644
index 0000000..29c0d42
--- /dev/null
+++ b/test/dedup.test.ts
@@ -0,0 +1,115 @@
+import {
+  generateIdempotencyKey,
+  isKnownKey,
+  registerKey,
+  clearKeys,
+} from "../src/dedup.js";
+
+describe("generateIdempotencyKey", () => {
+  afterEach(() => {
+    clearKeys();
+  });
+
+  it("produces the same key for identical inputs", () => {
+    const params = {
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 1000n,
+    };
+    const key1 = generateIdempotencyKey(params);
+    const key2 = generateIdempotencyKey(params);
+    expect(key1).toBe(key2);
+    expect(key1).toMatch(/^[a-f0-9]{64}$/);
+  });
+
+  it("produces different keys for different amounts", () => {
+    const key1 = generateIdempotencyKey({
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 1000n,
+    });
+    const key2 = generateIdempotencyKey({
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 2000n,
+    });
+    expect(key1).not.toBe(key2);
+  });
+
+  it("produces different keys for different payers", () => {
+    const key1 = generateIdempotencyKey({
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 1000n,
+    });
+    const key2 = generateIdempotencyKey({
+      invoiceId: "inv-123",
+      payer: "GDEF456",
+      amount: 1000n,
+    });
+    expect(key1).not.toBe(key2);
+  });
+
+  it("produces different keys for different invoiceIds", () => {
+    const key1 = generateIdempotencyKey({
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 1000n,
+    });
+    const key2 = generateIdempotencyKey({
+      invoiceId: "inv-456",
+      payer: "GABC123",
+      amount: 1000n,
+    });
+    expect(key1).not.toBe(key2);
+  });
+
+  it("changes the key when a nonce is provided", () => {
+    const base = generateIdempotencyKey({
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 1000n,
+    });
+    const withNonce = generateIdempotencyKey({
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 1000n,
+      nonce: "abc",
+    });
+    expect(withNonce).not.toBe(base);
+    expect(withNonce).toMatch(/^[a-f0-9]{64}$/);
+  });
+
+  it("produces the same key for the same nonce", () => {
+    const params = {
+      invoiceId: "inv-123",
+      payer: "GABC123",
+      amount: 1000n,
+      nonce: "xyz",
+    };
+    expect(generateIdempotencyKey(params)).toBe(generateIdempotencyKey(params));
+  });
+});
+
+describe("key registry", () => {
+  afterEach(() => {
+    clearKeys();
+  });
+
+  it("returns false for unknown keys", () => {
+    expect(isKnownKey("unknown")).toBe(false);
+  });
+
+  it("returns true after registering a key", () => {
+    registerKey("my-key");
+    expect(isKnownKey("my-key")).toBe(true);
+  });
+
+  it("clears all keys", () => {
+    registerKey("a");
+    registerKey("b");
+    clearKeys();
+    expect(isKnownKey("a")).toBe(false);
+    expect(isKnownKey("b")).toBe(false);
+  });
+});