Ben May

This plugin prevents imposters from leaving comments on your website with either the same name or email address as a registered user on your site.
This prevents users on the blog from being impersonated.

Duplicate Comment Authors

Prevents people posting comments using the same name as a registered users display name.

Frank Neumann-Staude

Ulf Schönefeld

polyres

This plugin/add-on requires the plugins FacetWP and Co-Authors Plus (WordPress plugin repository)!
This free plugin/add-on integrates FacetWP with Co-Authors Plus, allowing you to use Co-Author Plus custom fields for front-end search and filtering in a facet. 
With the help of Extend Co-Authors Plus for FacetWP, you can create a lable and choose the data source “co-authors” for your page or post.

Duplicate Comment Authors vs Extend Co-Authors Plus for FacetWP