def METHOD_NAME():
    """Test that linking already linked layers is a noop."""
    l1 = layers.Points(None)
    l2 = layers.Points(None)
    l3 = layers.Points(None)
    # no callbacks to begin with
    assert len(l1.events.opacity.callbacks) == 0
    # should have two after linking layers
    link_layers([l1, l2, l3])
    assert len(l1.events.opacity.callbacks) == 2
    # should STILL have two after linking layers again
    link_layers([l1, l2, l3])
    assert len(l1.events.opacity.callbacks) == 2