It always keeps the first of each duplicate group by default, so it you reverse-sort by location it will select things on C:\ for deletion over things on D:.
Where you have two or more things on C:, it will still select one of them. You may be able to use Edit > Select Pattern or similar (not sure what it was back in Opus 10) to de-select anything not on C:\ and/or select everything on C:\ if you wish to do it that way. (But remember that all of the duplicates may be on C:, so deleting all dupes on C:\ may delete all copies of some files.)