WMA link
Union
Union[{5, 1, 3, 7, 1, 8, 3}]
Union[{a, b, c}, {c, d, e}]
Union[{c, b, a}]
Union[{{a, 1}, {b, 2}}, {{c, 1}, {d, 3}}, SameTest->(SameQ[Last[#1],Last[#2]]&)]
Union[{1, 2, 3}, {2, 3, 4}, SameTest->Less]