I've been told many times that it's not allowed. It didn't seem to matter that I was only connecting in the offending duplicated city and not stopping, but was told I couldn't transit it multiple times. HUCA might work, since you might get an agent to do it, but I've been told 'no' many times.
The Aeroplan website will also not allow you to book it. If you try a simple multi-city itinerary, and choose some familiar destinations that force you to go back through the same city twice, the "select" button becomes greyed out on those particular flight combinations for the 2nd leg options that route through that city, if you've already selected a routing on the first leg that uses it.