Hi David @Blob Marley ,
Those Objects are Linked, so they are the same Objects in two different Collections.
What you can do, is to move non-‘duplicates’ into a new/different Collection and then right click the Collection with the Linked Objects and then Delete Hierarchy.
Tip: test this first on a new simple File. Otherwise, if something doesn’t work as expected, simply use CTRL+Z to Undo.
