Tests whether a list has no duplicated elements.
DuplicateFreeQ[list]
DuplicateFreeQ[list, test]
DuplicateFreeQ[{1, 7, 8, 4, 3, 4, 1, 9, 9, 2}]
→ FalseDuplicateFreeQ[{1, 2, 3, 4}]
→ TrueDuplicateFreeQ[{}]
→ TrueDuplicateFreeQ[{1, -1, 2, -3}, (Abs[#1] == Abs[#2]) &]
→ False