When importing files such as RIS or BIB into Papers, you have the option to automatically merge duplicate articles during the import process. Turning on duplicate merging can be helpful if you know you want to remove duplicates. Note, however, that doing so means the number of items you import into a library or list may not match the number of records in your BIB or RIS file, as merged items will be removed.
If you want to ensure all of the items in your RIS/BIB file are imported into your library or list every time, you may disable duplicate merging during. You may also merge duplicates at a later time if you choose this option.
You may also change the default behavior for duplicate merging during drag and drop operations in Settings: