When finding user by email, should return only single user

1) finding user by ```email``` will return a unique user, (as there is a uniqueness validation in place).
2) Also this would remove the requirement to do a ```.first``` at any place where the user is being looked up by email.

A question:
finding user by ```username```  here checks for size of ```users``` returned, is this the intended behavior ?, because there seems to be a validation in which checks for uniqueness of ```username``` .

Feedback?