VS2012 $(exists) only accepts scalar values

Question

Okay, this is more of a build error than a programming error. I have never had much reason to get my hands dirty with builds, so this error is baffling me.

I have tried

Answer 1

I had a similar issue in VS2013. I had no duplicates. The solution that worked for me was clearing the Visual Studio cache. http://blogs.msdn.com/b/willy-peter_schaub/archive/2010/09/15/if-you-have-problems-with-tfs-or-visual-studio-flush-the-user-cache-or-not.aspx