refactor: mark relay call signature as used

contracts/LSP6KeyManager/LSP6Errors.sol

@@ -165,3 +165,10 @@ error RelayCallBeforeStartTime();
  * @dev reverts when the period to execute the relay call has expired
  */
 error RelayCallExpired();
+
+/**
+ * @dev reverts when trying to use the same signature twice.
+ * @notice Cannot use signature: `signature` twice!
+ * @param signature The [EIP-191] signed data.
+ */
+error CannotUseSignatureTwice(bytes signature);

contracts/LSP6KeyManager/LSP6KeyManagerCore.sol

@@ -40,7 +40,8 @@ import {
     InvalidERC725Function,
     CannotSendValueToSetData,
     RelayCallBeforeStartTime,
-    RelayCallExpired
+    RelayCallExpired,
+    CannotUseSignatureTwice
 } from "./LSP6Errors.sol";
 
 import {
@@ -80,6 +81,9 @@ abstract contract LSP6KeyManagerCore is
     // https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v4.8/contracts/security/ReentrancyGuard.sol
     bool internal _reentrancyStatus;
 
+    // Mapping to keep track of all message hashes that have been approved by ALL REQUIRED owners
+    mapping(bytes32 => uint256) public signedMessages;
+
     mapping(address => mapping(uint256 => uint256)) internal _nonceStore;
 
     function target() public view virtual returns (address) {
@@ -347,6 +351,9 @@ abstract contract LSP6KeyManagerCore is
         uint256 msgValue,
         bytes calldata payload
     ) internal virtual returns (bytes memory) {
+        if (signedMessages[keccak256(signature)] == 1) {
+            revert CannotUseSignatureTwice(signature);
+        }
         if (payload.length < 4) {
             revert InvalidPayload(payload);
         }
@@ -403,6 +410,8 @@ abstract contract LSP6KeyManagerCore is
             _nonReentrantAfter();
         }
 
+        signedMessages[keccak256(signature)] = 1;
+
         return result;
     }