Even though I’m a speaker of American English, I’d most likely use the present perfect in that sentence. The use of the simple past is perfectly understandable, however.
The wording I’d most likely use would be this:
Haven’t we met before?
I think the most likely sort of situation in which I might use your simple past sentence instead would be in a situation where I had a specific idea in mind about exactly when I thought I had met you before (i.e. even though I only say ‘before’, I might actually have a specific time in the past in mind).
[color=darkblue]___________________________________ [size=75]“I am a part of all that I have met.” ~ Alfred Lord Tennyson[/size]
That would sound strange, Allifathima, though you might say ‘Did(n’t) we meet a long time ago?’
TIE - I agree with Esl_expert about what I’d say to someone I might have met previously. ‘We met before, didn’t we?’ is in colloquial, informal use in the area, so you have probably picked it up from hearing it, but it is not correct.
1a. I have been here before.OK 1b. I [color=red]had been here before. [color=red]Not acceptable unless there is something in the broader context that would justify the use of the past perfect.
2. [color=red]I had been here ago. [color=red]No
3a. I have been here sometime before.OK 3b. I [color=red]had been here sometime before. [color=red]Not acceptable unless there is something in the broader context that would justify the use of the past perfect.
[color=darkblue]_______________________________________________________________________ [size=75]“The most terrifying words in the English language are: I’m from the government and I’m here to help.” ~ Ronald Reagan[/size]