Recently we moved our projects to a new TFS server. This means that all workspaces link to the old TFS, and you will get the error message "The path [path] is already mapped in workspace [workspace]" when you try to add a new reference to a project you've already worked on. I keep running into this every once in a while, because we develop from virtual machines and we have different ones for different projects. I also keep forgetting what to do, so this is partly a message to self.

You can solve this by opening the Visual Studio command prompt and typing:

tf workspaces /remove:[projectname]

If you want to remove all of them (which is the case when switching to a new TFS), you can use a wildcard, like this:

tf workspaces /remove:*

Thanks to Gergely Orosz's post on the topic.