Mitigate conflicts or account repetions

I meet the following situation:

- there are accounts imported from my main site that are redundant... they point to the same person but they have different account names

I want to decide which one to remain:
1. if I choose the one from slave site, the account received from master site to be removed both from slave site and from master site, and be created on master site one account similar to the one from slave site
2. if I choose the one from the master site, the account slave site should be erased