I think that one set of cards will need you to farm different zones (say, 8 cards in the set is 8 different areas). There are more supporters than zones alone, so I think MANY cards will overlap.
Dunno if there won't be way too many of those. :D
Hm, I don't see the cards within a set as being distinct from each other, rather a set consists of x times the same
E: Although when thinking about it, it could actually be possible that GGG implements it in a way that you need the three different cards of the Divination Regal Orb set (being 1/3, 2/3 and 3/3) to complete the set. Getting a duplicate would then encourage a new kind of trading. (WTT DC Regal Orb 1/3 for 3/3)