I'm going to guess that only one of the accounts is going to be able to log in. (my guess is first match, '456' in your example)
Permissions in the file system are stored as the uid so you could chown files as either '456' or '789' and they would both show up as owned by 'quanba'. If you chown'd a file to 'quanba', again, i'm going to guess that it will resolve the uid as the first match in passwd ('456'). If you could manage to login as uid 789(not likely), you obviously wouldnt have access to the files owned by '456'.