Set doesn't consider equals() method

//  the Card class has equals() method implemented, so this works:
assert new Card("6s") == new Card("6s")

//  here's a list with 3 equal cards:
def list = ["4c", "6s", "6s", "6s", "6c"].collect {new Card(it)}

//  since three cards are equal, the following is pretty much expected:
assert list.clone().unique().size() == 3

//  but Set doesn't recognize cards as equal:
assert (list as Set).size() == 5