hi everyone, can you please help
i have random mint function and i need to eliminate the possibility of the same number appearing more than once. I'm trying to do it by iteration, and i know that bold part is in wrong place. I can't figure out how to make the code work in the right way

function mint() public payable {
require(msg.value == mintPrice, "Not enough ETH sent");

uint256 tokenId = _tokenIdCounter.current();
if (tokenId < 5) {
_tokenIdCounter.increment();
_safeMint(msg.sender, tokenId);
}
else {
_safeMint(msg.sender, _random(tokenId));
}

_owners[tokenId] = msg.sender;

}

function _random(uint tokenId) public returns (uint){
uint listLength = _existingTokenId.length;
bool alreadyExists = false;
for (uint i = 5; i if (_existingTokenId[i] == tokenId){
alreadyExists = true;
break;
}
if (!alreadyExists) {
_existingTokenId.push(tokenId);
}
}
return uint(
keccak256(abi.encodePacked(block.timestamp, block.difficulty, msg.sender))
) % tokenId;

}

thanks for answer