Error description: you see error page on website or in websitepanel website setting section.
Error contains details like this:
Config Error Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to ...
Config File \\?\D:\Websites\username\yourdomain.com\wwwroot\web.config
The error means you change some website setting in control panel and the change creates syntax error in web.config file.
To fix problem, you can remove <handlers> ... </handlers> in web.config file or delete web.config file to reset default setting.