This could easily happen, yes. Why don't we force contributions to be anonymous then?
Yes, there might be corner cases / race conditions where multiple people fix an issue at the same time, in that case I guess you should either choose the technically best solution or (if just as good) share the bounty. But I don't expect it to happen a lot.
I don't agree on forcing anonymous contributions or creating extra github accounts... You're making it very inconvenient that way.
You're right. After seeing how quickly the community learnt about and turned on the btcex owner, I am sure this wouldn't be a huge problem - but anonymizing things is a possible solution should it arise.