call out that foreign duplicated keys MAY be deleted

fixes w3c#446

spec/20-http_request_header_format.md

@@ -355,4 +355,4 @@ Following are allowed mutations:
 
 - **Add a new key/value pair**. The new key/value pair SHOULD be added to the beginning of the list. Adding a key/value pair MUST NOT result in the same key being present multiple times.
 - **Update an existing value**. The value for any given key can be updated. Modified keys SHOULD be moved to the beginning (left) of the list.
-- **Delete a key/value pair**. Any key/value pair MAY be deleted. Vendors SHOULD NOT delete keys that were not generated by them. The deletion of an unknown key/value pair will break correlation in other systems. This mutation enables two scenarios. The first is that proxies can block certain `tracestate` keys for privacy and security concerns. The second scenario is a truncation of long `tracestate`s.
+- **Delete a key/value pair**. Any key/value pair MAY be deleted. Vendors SHOULD NOT delete keys that were not generated by them. The deletion of an unknown key/value pair will break correlation in other systems. This mutation enables three scenarios. The first is that proxies can block certain `tracestate` keys for privacy and security concerns. The second scenario is a truncation of long `tracestate`s. Finally, vendors MAY also discard duplicate keys that were not generated by them.