It should be if each message includes an incrementing counter and the clients keep track of which counters they have already seen.
You've just added a nontrivial amount of memory usage as well that will look a lot like a memory leak as it grows.
you'll end up being protected from overflow for a practically-forever length of time with fairly few bits as long as the game isn't polling extremely fast.
it's not foolproof, and it's to thwart a kid, so ehh i'm not that into it.