Removed duplicate #includes.

  These includes were found via the new style-check:

        style/style-multiple-includes.t
  Added new style-test.

  This script tests that we don't include a header file more than
 once per source-file.

  For example this is bogus:

    foo.cc:
        #include "header.h"
        #include "foo.h"
        #include "header.h"

  (Because "header.h" is included twice.)