diff --git a/solidity/contracts/isms/hook/OptimisticIsm.sol b/solidity/contracts/isms/hook/OptimisticIsm.sol
index c81798929..3e22a01db 100644
--- a/solidity/contracts/isms/hook/OptimisticIsm.sol
+++ b/solidity/contracts/isms/hook/OptimisticIsm.sol
@@ -42,6 +42,10 @@ contract OptimisticIsm is IInterchainSecurityModule {
         uint8(IInterchainSecurityModule.Types.NULL);
 
     function verifyMessageId(bytes32 messageId) external {
+        require(
+            preverifiedMessages[messageId] == 0,
+            "OptimisticIsm: message already preverified"
+        );
         preverifiedMessages[messageId] = uint48(block.timestamp);
     }