A. It is two weeks since I've seen you.
B. It has been two weeks since I've seen you.
C. It is two weeks since I(last) saw you.
Would you please show me which of the sentences would be correct, and if they would mean the same thing?
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All three sentences are colloquially correct when spoken, and totally unambiguous. B would sound more formal because of the words "has been", but otherwise their meanings are all the same.
However, C is the only one which is correct in writing. The phrase "since I have done this…" is (I think) only properly correct when the word "since" is interpreted as "because", as in "Since I've eaten this pie, I'm not hungry any more". It sounds a bit clumsy even in this context.
Personally, in writing, I would prefer to mix B and C:
It has been two weeks since I (last) saw you.
And in speech, I would contract "it has":
It's been two weeks since I (last) saw you.
In speech and informal writing, the word "last" could optionally go at the end instead, but it reads a bit oddly in very formal writing:
It's been two weeks since I saw you last.
Just to add to Patrick's answer, you can't say "It is two weeks" because you are stating the fact that two weeks have passed since you saw the person so the two weeks are over. It has been two weeks is the way to go.