Since we started FastMail, our email server has used a system called “duplicate delivery suppression” to avoid mail loops.
Basically every email generated has a unique identifier in it’s header called the Message-Id. When an email is delivered to your account, we check if an email with the same Message-Id has already been delivered to the same folder and if so, we discard the message. This avoids an infinite loop problem if you accidentally setup forwarding between two systems to forward between each other.
Some people manage their email in a way this can cause problems however (eg cc themselves on a mailing list post, then they end up with a copy, and the copy being returned from the mailing list system is suppressed)
On the beta server at http://www.fastmail.fm/beta/, there’s no an extra per-folder option on the Options -> Folders screen that allows you to turn of duplicate delivery suppression on a per folder basis. In the vast majority of cases we recommend that people do NOT change this setting (one day you’ll accidentally setup an forwarding infinite loop and use up all your storage and bandwidth), but for those that do need it, it is now possible.