Visual Studio opens new instances on every click(!?)

I’m trying to use Visual Studio Express 2013 (for windows desktop). I chose the exe in the installed IDE folder, and it will launch ok when I double click a file in Unity.

Problem : Each file or error/warning I double click opens a new instance of VS!

How to fix this? Do I have to buy VS Professional?

I had this problem as well, I believe it was related to upgrading a project to a new version of unity. I resolved it by deleting the .csproj files and the .sln file for the project. Unity just recreated them immediately, and the files correctly opened in the same instance of VS.

I know it is a old question, but i got the same problem. Did your solved somehow by now?