I think the Mobile App and the Desktop App work in different ways. On the Desktop it just points to folders on your HD whereas iOS does not have this folder structure. So yes, it is duping it, however this means you can delete ones that are imported into LrM from the Camera Roll (wait for confirmation on this - or do a test of one that is backed up elsewhere).
Assuming you have CC account and are signed in, then what I believe should happen is that once connected to wifi (depending on your settings) LrM should sync to the Cloud and download the hi res to Lr Desktop and Optimise storage on the IPad (like Smart Previews for viewing).
if you haven't signed in (or have a different Adobe ID ) on the Lr Mobile I would make sure you photos are backed up elsewhere whilst you test it as there are many cases of sync issues caused by Catalog conflict.
If you don't have a CC acc then you should still be able to delete from the Camera Roll but photos will stay at Full Res within the App and they is no sync to the desktop (see other threads on here for discussion and requests for a solution to this).
Thanks for your response. That does seem to be what is happening, although I can't understand why Adobe would have designed it this way. I'll try your suggestion of deleting from Camera Roll - the annoying thing is that I don't have enough space to add all the photos to LR mobile at once, so I'll have to add some, delete them from Camera Roll, then add the rest. Not terrible, but an extra step that seems like it should be unnecessary.