Silently ignore redundant ON CONFLICT clauses in an UPSERT. Only the first

Silently ignore redundant ON CONFLICT clauses in an UPSERT.  Only the first
ON CONFLICT for each index is active.  Do not issue an error, since that might
break legacy queries.  But ignore the redundant ON CONFLICT clauses to prevent
problems such as described in [forum:/forumpost/919c6579c8|forum post 919c6579c8].

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