Items can't exist with the same IDs in the master DB. Therefore the only two scenarios that I can think of that would  produce duplicate items are:

1) somehow the introduction of second data source (i.e. file based which comes in SC10). But I have had time where item exists in both file and DB and this hasn't produced duplicate item.

2) a deserialisation issue where by a second entry is added to the index rather than replacing the old one. This is imo the most likely scenario, which should be solved by a simple reindex of the master index.

Please try reindexing master index and let me know if this resolves the problem.

lease can you try rebuilding the master index.