This is most likely caused by the local workspace cache for that machine. You can reset the cache with the following command (this might not be necessary):
tf workspaces /remove:*
And then repopulate it with:
tf workspaces /s:http://tfsservername:8080/tfs/Collection
Further info: http://blogs.msdn.com/b/buckh/archive/2006/09/12/path-is-already-mapped-in-workspace.aspx
And documentation for the workspaces command: http://msdn.microsoft.com/en-us/library/54dkh0y3.aspx