Thanks.  It was a CF card.  IIRC, PM adds a letter at end of filename if ingesting same file with incremental ingest off, right?  I didn't delete the prefs or delete cache folder.  
I had to manually delete each duplicate using the Gemini duplicate finding app the next day..  It was tedious process. 
I am trying to find out what triggered this as it happened at the very stressful moment when I was shooting pool on a late assignment.
Hi,
I was ingesting images during a long assignment the other day with incremental ingest checkbox on.  I was renaming the images after each ingest.  I was not formatting the card.  I discovered images that were already ingested were being reingested on subsequent ingests.  I haven't had this issue before and wondering what I might be doing differently.  I am on PM5 buildi 15451. 
Is the card write-protected?  If PM can't write its identifier to the card, it cannot track the files it has downloaded.  Or if you have reset PM's preferences/deleted its cache folder, the tracking information would be lost as well.
-Kirk