Because, the reason might be in a dump file. For example, you have enabled constraints in a target database, but they have been disabled in a source database. So, you could have entered duplicate values there, but those records will fail to import into a target table.
Hm. I didn't quite get it: why did you remove the original question?
Basically, it is a bad habit deleting/updating previous messages as this action makes the whole topic difficult to understand. OK, everyone from time to time says something stupid, but - who cares? - apologize in the next message or correct yourself and that's it.
Back to your problem: if you are SURE that duplicates do not exist in a source table, then - as far as I can tell - target table is not empty. As it is a very simple task - check whether it is empty or not, and you'll know the answer.
One more thing: is there a database trigger on a table? If so, what does it do?
@ebrian: I write small letters and it takes less time to finish a message. Try to practice and you'll see the result very soon. Heheheheh!