# A set of user IDs currently active
active_users = {'user_101', 'user_102', 'user_103'}

# Check if a user is active (O(1) average time complexity)
user_id_to_check = 'user_102'
is_active = user_id_to_check in active_users
print(f"Is {user_id_to_check} active? {is_active}")

# Add a new user
active_users.add('user_104')
print(f"Active users after adding: {active_users}")

# Attempt to add an existing user (no effect, sets only store unique items)
active_users.add('user_102')
print(f"Active users after adding existing: {active_users}")

# Remove a user
active_users.discard('user_101') # Use discard to avoid KeyError if item not present
print(f"Active users after removing: {active_users}")

# Remove duplicates from a list of IDs
duplicate_ids = ['item_A', 'item_B', 'item_A', 'item_C', 'item_B']
unique_ids = list(set(duplicate_ids))
print(f"Original IDs: {duplicate_ids}, Unique IDs: {unique_ids}")