Fastest way to merge two lists without duplicates

When developing SKILL scripts I find the need to join or merge two lists, while avoiding duplicate items.

E.g.:

list1 = '(a b c) 

list2 = '(a d c )

merge_func(list1, list2) -> '(a b c d )

Is there a fast/native way to do it in SKILL that is not the obvious iterative solution?