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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One way of thinking of this distinction is in terms of truth conditions. If the two are identical, they they will be true (or false) or appropriate (or inappropriate) in all the same conditions.
There is a condition under which strictly speaking the use of the past simple would be appropriate where the past perfect would be inappropriate. It is where the speaker is referring to someone (or thing) no longer living/existing. It sounds a little odd to say of a deceased person
I have known him for many years.
It would be as if he were still living. So, even though we would be understood if we used the past perfect, we should say
I knew him for many years.
But, in this case, the addressee is, ex hypothesi, still living, since s/he is the direct addressee. For that reason, it is not uncommon to hear "I met you before", particularly in the USA. You could think of it as a legitimate shortening, rather than a 'different tense'. One of the strange facts about languages with little inflection, is that grammatically grey areas arise more often
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.