suggestion - Access Keys
Sometimes its easier to get down & dirty with the Duplicate Files window itself & not use the Selection Assistant or context menu, i.e., use keyboard shortcuts.
Shortcuts for selecting all in /a group/ or all but one in /a group/ (possibly following some criteria defined in the Assistant?) or unmark group.
So if mark all except one in group were Ctrl+Shift+X, your actions could be something like:
Ctrl+Shift+X
|down arrow to next group
Ctrl+Shift+X
|down arrow, down arrow again (cause you want to skip the particular group)
Ctrl+Shift+X
!down, down, down, ...
Ctrl+Shift+X
Ctrl+Shift+X ...