Solved: “file into folder” + “forward” = “extra copy in inbox”
A long term and slightly annoying bug with FastMail has just been fixed.
Up until yesterday, if you went to Options -> Define Rules and created a "File into folder" rule and a "Forward" rule and an incoming email happened to match both rules, you would end up with a copy of the email in the file into folder (correct), forwarded to the specified email address (correct) and also a copy in your inbox (incorrect).
This problem has now been fixed. To enable it, you just have to go to Options -> Define Rules and click the Done button. That will rebuild your sieve script with the rule fix and stop the above problem occuring. More information is available in this forum post