.Net Build Error: The "exists" function only accepts a scalar value, but its argument "$(PackageSourceManifest)" evaluates to "[same path];[same path]" which is not a scalar value.

From: http://stackoverflow.com/questions/18006428/vs2012-exists-only-accepts-scalar-values

The solution which worked for me:

Open your .csproj file and look for any duplicated import tag.

In my case the .csproj had two lines like this:

<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio
\v10.0\WebApplications\Microsoft.WebApplication.targets" />

Remove one of them fixed the problem.