던 거 있다

that thing from before — is there one?

-던 points back at something from the past that is being brought up again, and 거 있다 either asks whether such a thing exists or flags one you both already know about. As a question it is a standard interview move: 해보고 싶으셨던 거 있었어요? 'was there anything you'd wanted to try?'. With 있잖아요 it opens a topic instead: 저번에 물으셨던 거 있잖아요 'you know that thing you asked me last time?'.

Register: polite conversational; interview-style