Eliminate duplicates in listing queryset

[hedleyroos]

If an object has the same category appear in "primary category" as well as "categories" then it appears twice in a listing.

[jbeyers]

Ack, just ran into that on UCM. Probable fix:

models.py, line 368:

- q = q.filter(q1|q2)
+ q = q.filter(q1|q2).distinct()

[qoda]

Be careful to use only() with distinct() as Oracle will fall over if not.

q = q.filter(q1|q2).only("id").distinct()

[qoda]

PR submitted to resolve this: #70

[hedleyroos]

I changed #70 a bit so we don't incur any performance penalties. Fix deployed to develop.