1) If I play a card that puts a character in play, like At the Gates, can I use it to put the card into play as a Duplicate? For example, if I have Maester Lomys in play, and I At the Gates, can I find another copy of him and play him as a Duplicate?

2) If Maester Lomys is in my dead pile, and I play at the Gates, am I still now allowed to play him, or do the rules for At the Gates take precedence?

1) Yes, you can do that.

2) No, you're not allowed to play or put into play a copy of a unique card that's already in your dead pile.

I hope OP doesn't mind me hijacking his thread a bit but if i have stuff that puts character into play like:

Ambush from the Plains or Ambush characters can i still use Lady Daenerys Chambers for it ?

Trigger for it is: "After you play a Targ character, ...."

Nope, you can't. You're not playing the character but putting it into play.

Thank you rules Genie.