Came across a new scenario for my play group last night, my brotherhood deck against a house bolton themed stark deck.

Reek made off with Beric who had dubious loyalty attached, so i played a new berric from my hand. My opponent then played imposter and winter cache on my berric that he controlled before i was able to use Reek to regain control of my beric him a turn later. (his plan was to use my berric to win challenges and then kill him for claim to leave me unable to replay him from my hand.)

So what happens to my original Berric and the attachments that my opponent put on him. I know that attachments remain in play under the control of the original player who gets to trigger responses etc (although passive effects still apply all the time?). We figured that the returning Berric would become a duplicate and the attachments would remain in play.

Your problem is a compeltely different one: You are not allowed play another copy of Beric if the first one is still in play, even if it is not under your control.

*looks abashed having reread the rules*

Nothing to see here folks, move along