When trying to map a local folder to a TFS server as a different user than the one under which I was logged in, I kept getting the message:
The working folder [source path] is already in use by the workspace [workspace name];[owner] on computer [tfs server].
I was finally able to map the folder by first deleting the local workspace as the logged in user:
tf workspace /delete /server:[tfs server] [workspace name];[owner]
Note that the "owner" was the currently logged-in user.