Sometimes it works, but then other times under identical circumstances it does not.
Therefore, it is a bug.
It has never worked reliably in all my history of cubase, which goes back to SX3.
Even with notes whose position and length parameters are absolutely identical, the doubles are sometimes not deleted.
And because it doesn’t tell you how many doubles were deleted, the only way to check is to peek behind each of the MIDI notes in turn, to see if the notes underneath have gone. Such a waste of time!