require implemente in setContent function

Requiere allows register a message only one time. If you try to register a message that it is already in the smart contract, the transaccion will fail.

Author: ARR552

Smart Contract/blockMask.sol

@@ -50,11 +50,12 @@ contract BlockMask {
     }
 
     function setContent (string memory _url, string memory _msg) public {
+       bytes32 url = keccak256(abi.encodePacked(_url));
+       
+       require(storedData[url].registrar == address(0));
 
        userContent[msg.sender].messages.push(_url) - 1;
 
-       bytes32 url = keccak256(abi.encodePacked(_url));
-       
        storedData[url].hashmsg = keccak256(abi.encodePacked(_msg));
        storedData[url].registrar = msg.sender;
        storedData[url].creationDate = block.timestamp;
@@ -94,4 +95,4 @@ contract BlockMask {
         return userContent[msg.sender].messages.length;
     }
 
-}
+}