Changed behavior of redirect in case of a duplicate message delivery or a mail loop.

If a duplicate is detected the implicit keep is canceled, as though the redirect was successful. This prevents getting local deliveries.
The original SMTP recipient is used when it is available to augment the entry in the LDA duplicate database. This way, duplicates are only detected when (initially) addressed to the same recipient.
The main goal of the duplicate detection is mail loop prevention, but this also has the effect that mere duplicate deliveries are handled specially whithout a good reason. We should fix this in a future version.