I think we've bumped into each other someday.
→ 어디선가 만난거 같은데요.
 
[예문]
A:You look familiiar to me
 낯이 익네요.

B : You too. Have we met before?
  저도 그러네요. 우리 본적있나요?

A : I think we've bumped into each other someday.
  어디선가 만난거 같은데요.

B : Wait a moment.. We've met at Kim's housewarming party.
  잠깐만요.우리 킴의  집뜰이에서 만났어요.

*비슷한 표현
Maybe we’ve met before.
전에 만난 적이 있는 것 같네요.

Don’t I know you from somewhere?
어디선가 만나지 않았나요?