diff --git a/contracts/WalletSimple.sol b/contracts/WalletSimple.sol
index 44a4929..eb4960f 100644
--- a/contracts/WalletSimple.sol
+++ b/contracts/WalletSimple.sol
@@ -80,6 +80,7 @@ contract WalletSimple is IERC721Receiver, ERC1155Receiver {
 
     for (uint8 i = 0; i < allowedSigners.length; i++) {
       require(allowedSigners[i] != address(0), 'Invalid signer');
+      require(!signers[allowedSigners[i]], 'Duplicate signer');
       signers[allowedSigners[i]] = true;
     }
 
diff --git a/test/gas.js b/test/gas.js
index 3d9b56f..9ec3d33 100644
--- a/test/gas.js
+++ b/test/gas.js
@@ -67,7 +67,7 @@ describe(`Wallet Operations Gas Usage`, function () {
       accounts[1],
       accounts[2]
     ]);
-    checkGasUsed(171030, transaction.receipt.gasUsed);
+    checkGasUsed(172810, transaction.receipt.gasUsed);
   });
 
   it('WalletSimple send [ @skip-on-coverage ]', async function () {