The unsorted union of a list S is a list containing the same elements as S but with the second and subsequent occurrence of any given element removed. For example, the unsorted union of the set {1, 3, 3, 2, 1, 3, 4} is {1, 3, 2, 4}. The unsorted union differs from the usual union {1, 2, 3, 4} in that the elements of the unsorted union are not necessarily ordered. The unsorted union is implemented in the Wolfram Language as DeleteDuplicates[list]. It can be implemented in the Wolfram Language top-level code as: UnsortedUnion1[x_] := Tally[x][[All, 1]]

disjoint union | union

DeleteDuplicates