Manual labor, or there might be a way to export them to a file and reimport them and then removed the duplicates?
But if they are journal articles or books available in a database (like pubmed or a university library database), it might be faster to download/import them from the database, and then run find duplicates.
But endnote can do the conversion, as long as it isn’t a two or three part surname or van der something when it is citing and creating the bibliography. The only complication comes if the same author is listed in two different ways and used in a bibliography and the ambiguous references settings treats the first author as different authors getting initials included. But you can turn that off, if you don’t happen to have any real distinct authors with the same surname.