How do I check for duplicates in Mendeley Reference Manager?

As of version 2.93 you will be able to review your duplicate references by viewing the 'Duplicates' smart collection, from here you will see duplicated references grouped into sets. You can delete any unwanted references to resolve a duplicate set.

Mendeley Reference Manager will also detect duplicates when importing new references, it will prompt you with a button that takes you to the Duplicates smart collection, if clicked, to review and resolve your duplicated references as necessary.