Make it an error if there's more than one build rule for a single output.

scripts/slave/compile.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """A tool to build chrome, executed by buildbot.
 
   When this is run, the current directory (cwd) should be the outer build
   directory (e.g., chrome-release/build/).
 
@@ skipping 861 matching lines @@
     if not goma_ready:
       assert options.compiler not in ('goma', 'goma-clang')
       assert options.goma_dir is None
 
     # ninja is different from all the other build systems in that it requires
     # most configuration to be done at gyp time. This is why this function does
     # less than the other comparable functions in this file.
     print 'chdir to %s' % options.src_dir
     os.chdir(options.src_dir)
 
-    command = [options.ninja_path, '-C', options.target_output_dir]
+    command = [options.ninja_path, '-w', 'dupbuild=err',
+                                   '-C', options.target_output_dir]
 
     # HACK(yyanagisawa): update environment files on |env| update.
     # For compiling on Windows, environment in environment files are used.
     # It means even if enviroment such as GOMA_DISABLED is updated in
     # compile.py, the update will be ignored.
     # We need to update environment files to reflect the update.
     if chromium_utils.IsWindows() and NeedEnvFileUpdateOnWin(env):
       print 'Updating environment.{x86,x64} files.'
       UpdateWindowsEnvironment(options.target_output_dir, env)
 
@@ skipping 461 matching lines @@
       sys.stderr.write('Unknown build tool %s.\n' % repr(options.build_tool))
       return 2
 
   options.target_output_dir = get_target_build_dir(args, options)
 
   return main(options, args)
 
 
 if '__main__' == __name__:
   sys.exit(real_main())