What does the notification event say for both these emails? You can find it on the Notification Audits of the task itself.
I guess out of the 2 notifications, one may corresponds to an Individual and other one to a Group.
However, it can be nail down by looking at the Notification Event in the Notification Audits (as suggested by Anusha) along with the body content of notification message.
Notifications audits is the best way to determine which notifications were sent to whom. Still, If all the basics are in place then you might want to check if notifications are active for multiple locales in ITSM. check the forms
NTE:CFG Notification Events
NTE:SYS Define NT Events
you can also use the NTE:Notifier Log form to get more information about the notifications which were sent out.