After importing or searching a reference in Endnote 20, I draged and released it to a group. Endnote 20 will create duplicated reference in that same group. Actaully, I had already setup ignore duplicated references.
Ignore duplicates only works when you are using the pubmed search that finds and automatically imports all the records into your open library. It is the search option that is a globe over a folder, and I would recommend you NEVER use it anyway. (and I believe this feature is gone anyway in Endnote20, so I really don’t have any idea what that button could do in Endnote20!).
The only otherway to discard duplicates during an import is when you import new records into a different empty library and then import that library into your main library using the “discard duplicates” available in the import menu.
Endnote20, despite numerous requests over the year, still has no duplicate check when you copy a record into your library. Only after the fact, you can run find duplicates.