Properly handle duplicated messages from the next epoch

Since 1fb9fdc3 we may attempt to buffer a record from the next epoch
that has already been buffered. Prior to that this never occurred.

We simply ignore a failure to buffer a duplicated record.

Fixes #6902

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7414)