Before this fix, a project subscription object was added twice to the list of subscriptions for a task when the user did not subscribe to the task directly. This caused the user to receive a comment notification twice for a given task.
This was probably a regression from efde364224.
Resolves https://community.vikunja.io/t/e-mail-notification-twice/2740/18
(cherry picked from commit 2c9becec101c14dd744444fffe244510e8394323)