Duplicates are common, particularly if you are importing records from a range of different databases. It is a good idea to regularly run EndNote's de-duplicator through your library. 

To find duplicates:

1. Click on the Library menu and select Find Duplicates
2. EndNote will scan your entire library for duplicate references. A reference is considered a duplicate if the author, title, and date match, or if the DOI is the same. 
3. EndNote will present each duplicate side by side. You will have to select the reference that you wish to keep; the other will be deleted.