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 property, VS2010 creates a message box telling me that "An item with the same key has already been added", and won't let me browse for an image.

I can add images for any other solution, even others ported from VS2005, but not this one. Any idea what I'm doing wrong?

Answer: 
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.
Running VS2010, I was able to fix this error by resetting the environment:

```
Tools > Import and Export Settings > Reset all settings

```