I have Visual Studio 2012 Package Project (from VS2012 SDK). I can't remember what I have done before, but at this moment I get a following error when I try to open source.extension.vsixmanifest file in designer mode: "An item with the same key has already been added". I am able to open this file only in "View Code" mode.
Moreover, what is weird, If I create another VS2012 Package Project in this Solution, the same error appears for the corresponding source.extension.vsixmanifest in the new project.
It looks like a dictionary or resources duplicated key problem. But I haven't modified any resources or dictionaries.
What is the reason of this error. How can I fix it?
I just had the same problem. On my System, the cause were projects with duplicate names that I created to have separate but similiar project templates for different Visual Studio versions. Renaming the projects solved it for me.