Making nickname non unique field

I want people just login with there e-mail adres en show everywhere on the page there full name.

I changed the login field already show its showing for email adres.

I changed the nickname field for extra spaces and let it show in the join page like full name.

The only problem is that more people have the same name. if somebody will try to join with a name which excist already then it will give a database error.

I think its not necessary to have the nickname field unique because you have a unique ID and a unique email address. so my idea is to take the unique key from the nickname field so persons can join with the same name.

Why i want this: I dont know people by nickname but i know them by real full name. Nickname is show everywhere on the site. photo's video's, between the menu ect..

I hope this is possible and somebody can help me with this.

Bye bye Michel