Save all used IPs to a file and bind it to the List.
Then to check if the current IP is unique try getting a line from the list, and as search criteria use your IP.
Check with "if" action If the result variable is empty. If it is, then this is unique IP, if it is not, then the list contains this IP.
If You got logic where its checked in loop then you will have a problem when you will get millions of ip to check in loop, it will simply take too much time. Then database way will be the best.