I'm not sure if this is of any help but I just deleted 2 dupes for a client. I felt pretty dumb because it took me about 45 mins to find the delete link. I realised 'Remove listing' must not be the same as deleting - you are just removing your ability to manage that page. Eventually realised I needed to be logged in as the owner to the dupe G+ and then click page setting and scroll all the way to the bottom to 'Delete'.
You probably already know this but just wanted to share to make sure if anyone else has the same issue as me they don't need to embarrass themselves by admitting what I've just admitted ;D
Honestly this can get annoying but then if it was easy I wouldn't have any clients.
Actually Adam, not quite sure what you meant but wanted to explain it's a little more complicated even.
LIVE dupes that you can see on maps you can't delete. Only support can.
Place pages, in the Places dash, if there are dupes you need to remove from account. If you say delete, from maps, (if there even still is that option in new dash), it won't delete them. Only support can. BUT you need to get them out of your account because it's a violation to have more than one claimed Place page and can cause many other problems. So remove listing.
You said "Eventually realised I needed to be logged in as the owner to the dupe G+ and then click page setting and scroll all the way to the bottom to 'Delete'."
Not sure but that part sounds like you were deleting a G+ page not a Place page???
If so, that's another animal! So you know dupe G+ pages all by themselves if not connected to a Place page aren't a problem per se and it's not a violation to have more than one and they don't rank or anything. (If it only has a follow button and no review option, it's a plain G+ page.)
But if it's merged or upgraded to a full G+ Page and Place page combo, then just deleting the G+ page may or may not delete the Place page (depending on which way it was created.) For instance if it was a combo page where the client or previous SEO manually set up a G+ page and manually merged it with Places, deleting the G+ page will still leave the Place page usually and it's the dupe Place page that's the problem as far as ranking and a violation.
ISN'T THIS FUN? And was that clear as mud???