Should I say :
1- I think I know you , I think we have met before.
2- I think I know you , I think we met before.
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Either choice could be used depending on the context.
Present perfect is used to cover events that happened any time before the present. I think we have met before implies that the meeting could have happened any time before now, and the speaker likely does not have a specific time in mind. The focus is on the state of having met.
Simple past is used to refer to a specific moment. I think we met before implies the speaker can recall a specific moment in the past. The speaker may follow this with the exact moment, e.g. I think we met before. It was at the..., and so on. The focus is on the moment of meeting.
If I did not plan to explain where the meeting happened, I would naturally go with the present perfect version. Depending on the speaker's intent, either could be used.