Just inform BrainBoss about this and they will take care of it. Keep in mind that many cheaters attempt to enter the bounty pool by using details of accounts from other users who may have also used that on other bounties. When I taken part in some bounties I found a user using my twitter account but then it wont help them because I would never be taking part in that bounty.

In either case our well-reputed managers know about such tricks and it is only a matter of verifying who owns which account using a signed message to confirm the real owner.

However in this case they are likely to be connected and then they will come up with an excuse of "thats my brother/sister/My Dog/Neighbor's Cat" and so on.