VS2010 “An item with the same key has already been added”

Question

I recently installed Visual Studio 2010 and copied and converted an old VS2005 solution to VS2010

When I edit this solution, if I try to change a control\'s .image p

Answer 1

Regarding Visual Studio 2015

The problem I was having was I could not load a project or re-add it to ANY solution. I would get the same error reported by others which is

Project '...' could not be opened because the Visual C# compiler could not
be created. An item with the same key has already been added.

This is going to sound overly simplistic, but when you get this error close ALL Visual Studio instances. Open the solution you were having trouble with and try again. This worked for me.