PeoplePress

It is currently in early stage of development and provides very basic functionality. 
We are looking forward to add more functionality with our upcoming releases.
Currently it offers login, password reset and a basic user profile with default fields.

It is lightweight user profile and account management plugin.

nafsadh

Validate Profile Credentials and Names
In a WordPress site users have multiple names in their profile, the user login, display name and nick name. In a multi-user site, there is often a need to keep them unique. You need to allow users have only credible names, such that,
<ul>
<li>All display names are unique</li>
<li>All nicknames are unique</li>
<li>No nickname is a display name or login name of another user</li>
<li>No display name is a nickname or login name of another user</li>
</ul>
Credible Names is a easy plugin to help you with that.

Credible Names

Ensure user's nickname and display name are unique and not a display name or nick name or user name of someone else

PeoplePress vs Credible Names