Duplicate username does not consider title case

I was able to register two accounts with usernames sabachanna and Sabachanna. It did not give duplicate username error. It is not able to capture lower case or title case differences. I tried to look into it but mongodb does not support case-insensitive queries.

Should we store the username as all capital or all lower in mongodb and then, on registration, convert the given username to upper case to see if duplicate is already available or not. Should we consider this on presentation layer?