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

scripts/slave/compile.py

Index: scripts/slave/compile.py
diff --git a/scripts/slave/compile.py b/scripts/slave/compile.py
index 3268c19a637b14e0ed8e0bdeec31d61ed62f523a..20beb0bfe22e2f4ad085152d19f3acaefd472647 100755
--- a/scripts/slave/compile.py
+++ b/scripts/slave/compile.py
@@ -879,7 +879,8 @@ def main_ninja(options, args):
     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.