When checking for duplicates EN compares Author Year Title and Reference Type fields - but you can be more specific or less specific - this is set up in Preferences - Duplicates.
Check for duplicate references in your EndNote library.
Click on References - Find Duplicates. Pairs of duplicates are displayed side-by-side allowing you to compare the data between them - transfer any data from one to the other and choose which one to keep. The earlier reference is the one on the left.
Click on Skip to go through them one at a time or Cancel to return to a list of duplicate references with the duplicates highlighted - the first entered reference is not highlighted - the other duplicates are highlighted. The duplicates can be deleted from here - these are moved to Trash.