Most languages provide set data structures. Hash sets use hash tables for $O(1)$ average insertion and lookup. Tree sets use balanced trees for $O(\log n)$ operations with sorted order.
Sets automatically handle uniqueness. If you add $3$ twice, the set still contains $3$ only once.
Common operations like union, intersection, and difference have built-in methods in most standard libraries. Use sets when you need to track unique items or check membership quickly.