Previously, users without the Manage all tasks role operation were able to self-assign tasks that were already assigned. Users had to make sure to manually refresh the Unassigned task list before using the Assign to me feature, to avoid this scenario. Now, if a user tries to self-assign a task that's already assigned, they're presented with a notification informing them that the task has already been assigned. If you have the Manage all tasks role operation, you're presented with the option to Reassign.