When you merge a set of duplicate photos, Photos will keep the best version in the library, where it is and move the discarded duplicates to the Recently Deleted album. The Recently Deleted album will automatically be emptied after 30 days. You should check, what you are currently having in Recently Deleted to be able to recover discarded duplicates, that you want to keep.
The Trash (or garbage bin) in the Finder, at the end of the Dock, is independent of the Recently Deleted album in Photos. Items you delete in Photos for Mac will never appear in the Trash on your Desktop. You can empty it without removing items from Photos. This is different from the way it used to be in iPhoto or Aperture, where deleted photos have appeared in the Trash on the Desktop
See: Remove duplicate photos in Photos on Mac – Apple Support (UK)