Can someone suggest a place to start - or a cause for this error?

When I try to edit a member profile, or change my member services, or edit a host login - a similar error is created.

I've deleted the .net temporary files and restarted the site and server, checked the file dates for the latest dnn version 7.3.1 are in place. Googled... hmmm...

Here's the error:

InnerException:d:\...sitefolder...\DesktopModules\Admin\Security\EditUser.ascx.cs(622): error CS0433: The type 'DotNetNuke.Modules.Admin.Security.MemberServices' exists in both 'c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\e66e1464\495cea0b\App_Web_xlahalrp.dll' and 'c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\e66e1464\495cea0b\App_Web_nrzh5sdw.dll'