SWI-Prolog -- is

Availability::- use_module(library(lists)).(can be autoloaded)

[semidet]is_set(@Set)

True if Set is a proper list without duplicates. Equivalence is based on ==/2. The implementation uses sort/2, which implies that the complexity is N*log(N) and the predicate may cause a resource-error. There are no other error conditions.