I believe we've met.
a phrase suggesting that one has already met a person to whom one is being introduced. John: Alice, have you met Fred? Alice: Oh, yes, I believe we've met. How are you, Fred? Fred: Hello, Alice. Good to see you again. Alice: Tom, this is my cousin, Mary. Tom: I believe we've met. Nice to see you again, Mary. Mary: Hello, Tom. Good to see you again.