了…了【le … le】

Two 了 in one sentence say the action has run for a stretch of time and is still running. The first marks the completed portion, the second says that is where things stand now.

One 了 or two

Drop the second 了 and the stretch is over.

The first sentence is about something finished; the second is about something in progress. English marks this with the perfect, and it is the same distinction.

With a quantity instead of a duration

The implication is the same: three so far, and the evening is not over.

还 and 已经 sit comfortably with it

Negation breaks the pattern

A stretch of not doing something uses 没 and takes neither 了.

The final 了 survives here because the state still holds; the verbal 了 cannot, because 没 forbids it.