1. It's been a long time.

2. It's been a long time since we've met.
3. It's been a long time since we met each other last.
4. It's been a long time since I met you last.
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Are they all the same in meaning?
Does #1 mean #2, #3, or #4?

I'd change 'met' to 'saw' in 3 and 4. Then they'd all mean close to the same thing.