Check duplicate extensions before processing.

ClientHello and ServerHello are not allowed to include duplicate extensions.
Add a new helper function to check this and call as appropriate. Remove ad-hoc
per-extension duplicate checks which are no unnecessary.

Add runner.go tests to verify such message correctly rejected.

Change-Id: I7babd5b642dfec941459512869e2dd6de26a831c
Reviewed-by: Adam Langley <>
8 files changed