diff --git a/contracts/src/Chainvoice.sol b/contracts/src/Chainvoice.sol
index d2cd5219..7e85f878 100644
--- a/contracts/src/Chainvoice.sol
+++ b/contracts/src/Chainvoice.sol
@@ -312,7 +312,9 @@ contract Chainvoice {
 
         uint256 totalAmounts = 0;
         uint256 totalNativeFee = fee * n;
-        // Validate and sum
+        // Validate and mark paid in the same pass so a duplicate id in the
+        // batch hits AlreadySettled() on its second occurrence, before any
+        // funds move.
         for (uint256 i = 0; i < n; i++) {
             uint256 id = invoiceIds[i];
             if (id >= invoices.length) revert InvalidInvoiceId();
@@ -324,11 +326,7 @@ contract Chainvoice {
             if (inv.tokenAddress != token) revert MixedTokenBatch();
 
             totalAmounts += inv.amountDue;
-        }
-
-        // Effects: mark all paid & bump fee accumulator BEFORE interactions
-        for (uint256 i = 0; i < n; i++) {
-            invoices[invoiceIds[i]].isPaid = true;
+            inv.isPaid = true;
         }
         accumulatedFees += totalNativeFee;
         // Interactions
diff --git a/contracts/test/Chainvoice.t.sol b/contracts/test/Chainvoice.t.sol
index 0d45976c..8b2733ae 100644
--- a/contracts/test/Chainvoice.t.sol
+++ b/contracts/test/Chainvoice.t.sol
@@ -196,6 +196,23 @@ contract ChainvoiceTest is Test {
         assertEq(alice.balance, aliceStart + totalPrincipal);
     }
 
+    function testPayInvoicesBatch_RevertOnDuplicateId() public {
+        vm.prank(alice);
+        chainvoice.createInvoice(bob, 1 ether, address(0), keccak256("dup1"));
+
+        uint256 fee = chainvoice.fee();
+        uint256 totalFee = fee * 2;
+        uint256 totalPrincipal = 2 ether;
+
+        uint256[] memory ids = new uint256[](2);
+        ids[0] = 0;
+        ids[1] = 0;
+
+        vm.prank(bob);
+        vm.expectRevert(Chainvoice.AlreadySettled.selector);
+        chainvoice.payInvoicesBatch{value: totalPrincipal + totalFee}(ids);
+    }
+
     /* ------------------------------------------------------------ */
     /*                       FUZZ TESTING                           */
     /* ------------------------------------------------------------ */