It all depends on the email being sent. With a catchall setup, duplicate emails - to Gossamer Mail, duplicate emails are emails which have the same headers (sort of) - will not be delivered during each run of incoming.pl. So when you're sending mail to yourself from Gossamer Mail with To as well as Cc, two emails will get sent to the mail server. However, depending on your mail server, you the headers of the email may be different. If they are, you will get two copies. If they're the same, then you'll only get one.

It isn't normal behaviour to only deliver one of the emails, but this behaviour is a side effect of having to support mail servers that do not add a Delivered-To (or equivalent) header.

Adrian