Maybe there should be a way of reporting copied ideas, then they could match it against the database from the previous week to see if there’s any validity to this claim.

If people “know it’s copied” and downvote the idea, then there’s an even bigger problem with the system. Voting is supposed to be anonymous, and if a large percentage of voters know who submitted a forum idea, and change their vote based on that, then it’s by no means a stretch of the imagination to suppose that their original votes were based on the submitter.