I have a hard time believing double Moneylender with IGG is a good way to go. If you buy a Silver instead, any hand you draw Moneylender-Copper, you get $3, same as Silver-Copper. Trashing the Copper isn't that great, since you actually like having tons of copper in the endgame. And that's if you happen to get a random $4 hand; if you pay $5 to get the second Moneylender, there's the huge opportunity cost.
It's not uncommon for there to be multiple Feasts in your deck at once. It's usually not how you plan to use that card, but hey, stuff happens. It's definitely not a card you never want to buy more than one of.
What about Contraband? I imagine if I think long enough I can come up with some situation where I want multiples, but it seems pretty rare.