The issue is caused by more than one player contributing/completing a task at the same time (or very close together), causing the game to send a message to the whole Guild that the task has not been received. We performed extensive investigation on game accounts for the issue and discovered that the tasks rewards are being received.
You got the rewards the first time and the “already received” mail the second time.
The issue may also be caused by you changing guilds (but that is actually a different problem which is by design, this was an actual issue). If it happened recently you can check in your ‘Collected’ mail to be sure you got the rewards. It is fixed now!
We have caught up! For now