If you have imported references on the same subject from more than one online resource, you may find that you have some duplicate references in your library. It is important to remove duplicates if you intend to use EndNote with Word to cite references, to avoid errors in your bibliography.
EndNote will search the currently selected references for duplicates. To search the whole EndNote Library, click on the All References folder.
To catch any outstanding duplicates it can be helpful to manually look through your library.
You can change the sensitivity of EndNote’s duplicate detection: