Is Delete Double broken? It works for individual selections but not for a larger selection.
Thank you
I’m experiencing the same issue - annoying !
There are forum reports on issues with Delete Doubles that dates back to 2011.
I am assuming the developers are aware.
I too have this issue!
I inadvertently played and recorded from a controller keyboard that was simultaneously attached via MIDI and USB, so the notes and their MIDI channels are absolutely identical. List Editor confirms this, right down to their timing and length.
I’ve been using this feature successfully on-and-off since Cubase on the Atari 1040 without problem; now it is doing absolutely nothing (Cubase v12.0.50, build 387).
It’s looking like Cubase (and Steinberg) is leaving me little option but to attempt to re-record the piece. 
Anyone else any ideas how I can delete these double notes en masse, given they are entirely identical? Anything ‘fancy’ worth trying in Logical Editor, for instance? I can’t think of an approach…
Steve.
No genius solutions here I’m afraid, and fortunately this feature works fine for me, but I’d be trying things like duplicating the track and seeing if it works on the duplicate, or importing the track into a new project and trying there etc. If as some state it works for individual notes but not for larger selections perhaps macros can helps speed up the process, but I’m guessing here. So sorry this may be a useless post. Consider it a bump until one of the resident experts jump in! 
Many thanks for the suggestions!
It looks like great minds think alike as I’ve tried these(!), including chopping the track up into much smaller parts and working on those, moving to other tracks and changing the MIDI channel en masse to ‘force’ the issue of these being duplicates.
I also tried re-duplicating the notes (so there were now 4 duplicates) and applying the Delete Doubles function to that. Funnily enough, it only appeared to find the ‘new’ duplicates, deleting 2 of the 4!
How very strange.
Thanks again,
Steve.