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If a json5 file for make_names.py contains duplicated names, make_name.py
produced invalid C++ code and we had to remove duplicates.

* make_names.py should report an error for duplicated entries instead of
  producing invalid code.

* html/keywords.json5 should allow duplicated entries because it
  contains keyword sets for multiple attributes and some keywords are
  valid for multiple attributes.

Change-Id: I81c2a4a978ece5a03471fd8b37d85b61cda2a8e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032465
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737211}