I would like to know which one is correct when the structure refers to a past event.
Past Perfect + Past Simple - It had been a long time since I met her.
Past Perfect + Past Perfect - It had been a long time since I had met her.
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Sign up to join this communityEither could be correct, depending not on what happened, but on how it is seen, looking back.
It matters not whether the past event was yesterday or 15 years ago. It matters only how that event relates to the situation in which the relevant participants find themselves today, or what has happened in between, or some combination of the two.
As to how the structure refers to a past event, does anyone doubt 'It had been a long time since I had seen her' is grammatically identical to '… since I had met her' and makes no possible difference?
There seems to be a single event; a meeting, not a first and any number of subsequent events and either way, there is no problem.
What tense described the less recent past does not constrain the choice of tenses available for the even-less-recent past.
If the whole sentence, that is the main-clause and the since-clause, is in the past then either Past Simple or Past Perfect is possible.
However, generally, Past Simple is more common.
If you place it in the present the only Past Simple is possible: