If the project is already done and released, there's a good chance a quick search will find it. If someone is in the middle of it but never finished it, it's fair game.
This is the main reason I usually start a Personal Projects page early on, as a way of saying "hey, I'm doing this, so don't try it unless I say I've given up - and if anyone knows that someone is already doing it, tell me!"
I know there's been occasions when someone releases a great translation and someone pops up saying "aw, I was working on that for ages
", but I figure that such people, sad as it is, should've gone more public with their efforts, or got on with it. You snooze, you lose.