Let's see,
!a[$0]++ first
a[$0] we look at the value of a[$0] (array a with whole input line ($0) as key).
If it does not exist ( ! is negation in test will eval to true)
!a[$0] we print the input line $0 (default action).
Also, we add one ( ++ ) to a[$0], so next time !a[$0] will evaluate to false.
Nice, find!! You should have a look at code golf!
