Putting a card into play like At the Gates does is not the same as playing a card. Also At the Gates doesn't say anything about a unique maester at all and you would not be able to put into play a unique Maester you already control.
I'm not following this. Are you saying that At the Gates doesn't allow you to get a unique Maester from your deck? That's simply not true. So long as the card is an in-House or neutral Maester of the correct printed cost, it meets the requirement. The fact tha At the Gates says nothing about "unique" means that it is not checked one way or another, not that it prevents
you from using it to get a unique Maester.
Further, as a "put into play" effect, it can be used to dupe a unique card that you already have in play - provided that the card you "put into play" meets all the requirements for the effect (in-House or neutral Maester character, printed cost 3 or less).
The fact that At the Gates can put unique Maesters into play - and dupe unique Maesters already in play - has been pretty well established for a long time.
That said, you still cannot use "setup" to dupe the location you get with House of Dreams because the rules specifically say you cannot place dupes during setup. In order to place dupes during setup, you would need to put down 2 of the same unique cards so that when you reveal your setup cards, one enters play as a duplicate. That means setup cards cannot enter play as duplicates, period, blocking you from duping the House of Dreams location with a setup card.