GN: Throw an error for duplicate object files.

tools/gn/ninja_binary_target_writer.h

Index: tools/gn/ninja_binary_target_writer.h
diff --git a/tools/gn/ninja_binary_target_writer.h b/tools/gn/ninja_binary_target_writer.h
index fe7a132c9baa38d9aed9e7c1bbc7d18c4b5cfa0d..5888ac59bbac22c0ddda4788c2f12ca0adebc1e3 100644
--- a/tools/gn/ninja_binary_target_writer.h
+++ b/tools/gn/ninja_binary_target_writer.h
@@ -116,6 +116,10 @@ class NinjaBinaryTargetWriter : public NinjaTargetWriter {
   // tool type. The tool must support precompiled headers.
   OutputFile GetWindowsPCHFile(Toolchain::ToolType tool_type) const;
 
+  // Checks for duplicates in the given list of output files. If any duplicates
+  // are found, throws an error and return false.
+  bool CheckForDuplicateObjectFiles(const std::vector<OutputFile>& files) const;
+
   const Tool* tool_;
 
   // Cached version of the prefix used for rule types for this toolchain.