How to restrict two people log in with the same user credentials at any given time - Rails

I am researching on how to add protection to an existing rails app. The protection i need is to restrict two people log in with the same user credentials at any given time.

What would be the best approach around this?

I haven't seem to find any gem or something in particular.

Note that i am not using Devise but custom user authentication so please do not suggest anything related to devise.