This addon does what it should, has lots of options, but most importantly, it allows you to define the deletion rules along with which bookmark folder to apply them on, which other extensions don't do. This allows you to delete duplicates in just about any way you wish.

For example, you can remove the duplicates only in one folder that you specify, or you can select the folders in which duplicate bookmarks will not be deleted, while deleting them everywhere else. Very useful.