Bounty manager should check carefully all participants because some have many accounts in one bounty.Just look for a bounty that has a big allocation with few participants just make sure that the project is promising and legit...Also look for a bounty that has a limited participants...
I agree with you on this because it can help reduce the rate of fraud in bounty and multiple account by one participant will be reduced. You should do a very good research about a project and check how crowded the community is before you join the campaign and make sure the manager has has good reputation.