I have X users ID and some of them are being used by more than 1 person.

A ......... ......... B
UserId01 ....... Person01
UserId01 ....... Person02
UserId02 ....... Person03
UserId03 ....... Person04
UserId03 ....... Person05

What do I need? Count how many UserId I have, how many people I have and if the numbers are equal so there's person per userid. But if they're different 2 or more people are using the same userid.
That's why I need to compare A and B columns in order to list these people that are using same userid.

Example: UserId03 is being used by Person04 and Person05.