Groovy: find all elements in a list equal to the max element
How you've done it for the simple example is exactly how I would do it.  I may use groupBy if I was dealing with a more complex object.

    List v = [ 1,2,3,4,5,5 ]
    def max = v.max()
    def results = v.groupBy {it}.get(max)
    assert [5,5] == results