'X' is ambiguous in the namespace

I upgraded my projects with the latest version of a third party control (happened to be Telerik, but I've seen the problem occur other times as well) and got the ominous error on the BUILD:

 'Filename' is ambiguous in the namespace.

The path led to the temporary folders directory. I've read a post where someone deleted all the files in the directory. But that was after I already fixed it.

 The fix is simple, use REBUILD instead of BUILD.

 BTW, I'm using Visual Studio 2010 and Framework 4.0.

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