A rollout of some new code today contained some errors that badly affected two separate areas of FastMail.
1. Errors selecting any emails to action on the classic interface would cause a fatal error
An internal misuse of an API meant that selecting any emails on the classic interface Mailbox screen and trying to action those emails would fail with a fatal error. Reading and applying actions to individual emails continued to work fine. The way this manifested itself unfortunately wasn't picked up in our testing before being rolled out. It has been fixed now and we'll update our tests to catch this. The error lasted for for about 3 hours.
2. Pop links for some users re-downloaded all emails again
A long term bug in the pop retrieve system resulted in a very rare and intermittent problem where some links for some users would forget all existing downloaded message. This means that in certain cases users that had set the "Leave on server" option might see existing messages that had been downloaded previously downloaded again, possibly several times. This obviously resulted in duplicate copies of the same message appearing in a folder.
Unfortunately a fix to this bug rolled out to one server for a short time actually made things worse, causing the same problem to occur for more users than the original bug.
A correct fix for the original problem and the subsequent bug has now been rolled out everywhere.
Users affected by this bug can find and remove any duplicate messages using the Advanced -> Folders -> Mass delete/Download/Remove duplicates ... (button down the bottom) screen. Select the folder with the duplicates at the top, and use the "Remove duplicate messages" section to find and remove any duplicate messages.