diff --git a/docs/INTEGRATION.md b/docs/INTEGRATION.md
index a0aa118..19f94e4 100644
--- a/docs/INTEGRATION.md
+++ b/docs/INTEGRATION.md
@@ -127,6 +127,7 @@ Notes:
 - Base-asset wallets pay over LND; Taproot Asset wallets over litd — the SDK
   picks the endpoint from the wallet's asset automatically.
 - Wrong password → `DecryptionError`. Node-side failure → `PaymentSendError`.
+- If the invoice was already paid (duplicate, or a replayed `idempotencyKey`), the SDK detects the returned `COMPLETED` transaction and resolves with `payment.status === 'SUCCEEDED'` without re-paying on the node. `payment.paymentPreimage` is `undefined` in this case.
 
 **Sandbox wallets** need no password and no node — the backend settles the
 transaction asynchronously and `payment` resolves `null`. Control the outcome
diff --git a/packages/payments/README.md b/packages/payments/README.md
index d409ee9..90eb38d 100644
--- a/packages/payments/README.md
+++ b/packages/payments/README.md
@@ -204,6 +204,8 @@ Base-asset wallets pay over LND; Taproot Asset wallets pay over litd — the SDK
 selects the endpoint automatically from the wallet's asset. A wrong password
 throws `DecryptionError`; a node-side failure throws `PaymentSendError`.
 
+If the invoice was already paid (a genuine duplicate, or a replayed `idempotencyKey`), the backend returns the existing `COMPLETED` transaction instead of creating a new one; the SDK detects this and resolves immediately with `payment.status === 'SUCCEEDED'` without re-paying on the node. `payment.paymentPreimage` is `undefined` in this case — the transaction record doesn't store it.
+
 **Sandbox wallets** need no node, no macaroon, and no password — just call
 `send` and the backend settles the transaction for you. `payment` comes back
 `null`; observe the outcome via webhooks or by polling the transaction status.
diff --git a/packages/payments/src/resources/transactions.send.test.ts b/packages/payments/src/resources/transactions.send.test.ts
index 6b26498..6c78f9a 100644
--- a/packages/payments/src/resources/transactions.send.test.ts
+++ b/packages/payments/src/resources/transactions.send.test.ts
@@ -38,7 +38,11 @@ async function startNode(lines: object[]): Promise<string> {
 }
 
 /** Fake GraphQLClient that answers the operations send() issues. */
-function fakeClient(restHost: string, environmentType: 'LIVE' | 'SANDBOX' = 'LIVE'): GraphQLClient {
+function fakeClient(
+  restHost: string,
+  environmentType: 'LIVE' | 'SANDBOX' = 'LIVE',
+  createSendTransaction: object = { id: 'tx1', status: 'PENDING', payment_request: 'lnbc1xyz' },
+): GraphQLClient {
   const masterKey = deriveMasterKey(PASSWORD, TEAM_ID);
   const encrypted_symmetric_key = nip44Encrypt(SYMMETRIC_KEY, masterKey);
   const encrypted_macaroon = nip44Encrypt(MACAROON_HEX, SYMMETRIC_KEY);
@@ -87,7 +91,7 @@ function fakeClient(restHost: string, environmentType: 'LIVE' | 'SANDBOX' = 'LIV
       return {
         payment: {
           transaction: {
-            create_send: { id: 'tx1', status: 'PENDING', payment_request: 'lnbc1xyz' },
+            create_send: createSendTransaction,
           },
         },
       };
@@ -166,4 +170,49 @@ describe('Transactions.send', () => {
       /admin macaroon/,
     );
   });
+
+  it('short-circuits an already-COMPLETED transaction without paying on the node', async () => {
+    // Point at an unroutable host so any accidental node call fails the test.
+    const transactions = new Transactions(
+      fakeClient('http://127.0.0.1:1', 'LIVE', {
+        id: 'tx1',
+        status: 'COMPLETED',
+        payment_hash: 'ph-existing',
+        fee: '3',
+        payment_request: 'lnbc1xyz',
+      }),
+    );
+
+    const result = await transactions.send({
+      walletId: 'w1',
+      password: PASSWORD,
+      destination: { bolt11: 'lnbc1xyz' },
+    });
+
+    assert.ok(result.payment);
+    assert.equal(result.payment.status, 'SUCCEEDED');
+    assert.equal(result.payment.paymentHash, 'ph-existing');
+    assert.equal(result.payment.feeSat, '3');
+    assert.equal(result.payment.paymentPreimage, undefined);
+    assert.equal(result.transaction.status, 'COMPLETED');
+  });
+
+  it('still executes the node payment for a non-completed transaction', async () => {
+    const host = await startNode([
+      { result: { status: 'SUCCEEDED', payment_hash: 'ph', fee_sat: '1' } },
+    ]);
+    const transactions = new Transactions(
+      fakeClient(host, 'LIVE', { id: 'tx1', status: 'PENDING', payment_request: 'lnbc1xyz' }),
+    );
+
+    const result = await transactions.send({
+      walletId: 'w1',
+      password: PASSWORD,
+      destination: { bolt11: 'lnbc1xyz' },
+    });
+
+    assert.ok(result.payment);
+    assert.equal(result.payment.status, 'SUCCEEDED');
+    assert.ok(lastBody, 'node should have been called');
+  });
 });
diff --git a/packages/payments/src/resources/transactions.ts b/packages/payments/src/resources/transactions.ts
index f95400d..7b27068 100644
--- a/packages/payments/src/resources/transactions.ts
+++ b/packages/payments/src/resources/transactions.ts
@@ -146,6 +146,29 @@ export class Transactions {
       input: buildCreateSendInput(params),
     });
     const transaction = createRes.payment.transaction.create_send;
+
+    // Already-completed: `create_send` found an existing COMPLETED transaction
+    // with the same payment hash (a genuine duplicate, or an idempotency-key
+    // replay) and returned it instead of creating a new one. Paying it again
+    // on the node would fail (or double-pay), so short-circuit and report it
+    // as the successful send it already is.
+    if (transaction.status === 'COMPLETED') {
+      return {
+        transaction,
+        payment: {
+          status: 'SUCCEEDED',
+          paymentHash: transaction.payment_hash ?? undefined,
+          // `fee` is already sats today (backend writes it from LND's
+          // `safe_fee`) — same unit as `feeSat`, so no numeric conversion.
+          feeSat: transaction.fee ?? undefined,
+          // PaymentsTransaction has no preimage field, so it can't be
+          // recovered for an already-settled transaction — only a payment
+          // actually executed on the node below returns one.
+          paymentPreimage: undefined,
+        },
+      };
+    }
+
     if (!transaction.payment_request) {
       throw new PaymentSendError('Backend did not return a payment request.');
     }