Finding all the users that have duplicate names

I have users which has first_name and last_name fields and i need to do a ruby find all the users that have duplicate accounts based on first and last names. For example i want to have a find that will search through all the other users and find if any have the same name and email. I was thinking a nested loop like this

User.all.each do |user|
 //maybe another loop to search through all the users and maybe if a match occurs put that user in an array
end

Is there a better way