As you add references to your EndNote library, you may inadvertently add duplicate records, or multiple versions of the same citations, with minor variations. By default, EndNote identifies a record as a duplicate if two or more records are assigned the same reference type (such as Journal Article), and include the same information in the Author, Year and Title fields. You can change the criteria for determining duplicates through Edit > Preferences > Duplicates.
To check for duplicate references in your EndNote library click on Library > Find Duplicates.
Pairs of duplicates are displayed side-by-side allowing you to compare the data between them. The earlier reference is the one on the left. You can make changes to either reference and choose which one to keep.
For each set of duplicates you have the option to:
EndNote 20 Find Duplicates (1:02)