Why I don't think Exclude Duplicates works as it should...

There is an option on Import to Exclude Duplicates.  The idea being that if you are importing from the same directory multiple times, pictures already imported won't appear again.  So far, so good.
There are two problems. Firstly it takes Capture One a noticeable time to check each photo against the catalogue, so in most cases you're better off just picking the photos you want from the full list (presumably the last ones ordered by date, where you have added more photos since last importing. In other situations I suspect importing everything and sorting them out within Capture One would be more efficient )
Secondly if you do filter out photos and then import all the non-duplicate photos, Capture One still tries to import the duplicates and reports them as such - so you could get the same effect by just importing all without the exclusion...

Is there something I'm missing?