No, it doesn't mean that the combination is unique, it means that
:login and :email are going to be unique.
There are two possible reasons, the user was added manually at the
database (which is bad, as the database should have a unique index at
both columns) or you got into a racing condition, where the two
selects hit the database at the same time and thus both returned
false.
Best thing to do is to improve you database schema by creating some
unique indexes that reflect your validations.
In ActiveRecord::Base, the save method takes a flag called
perform_validation, which you can set to false. Check your controller
code to make sure someone didn't decide to skip the validations
somewhere.