Imagine you're developing a real-time voting system, where each user can only vote once but you need to efficiently check if a user has already voted. What ES6 data structure would you implement for storing user IDs?

For a real-time voting system, a Set would be the best choice. A Set only allows unique values, making it efficient for storing user IDs and quickly checking if a user has already voted, ensuring each user can only vote once in the system.