When domain subscription is created in PBA two notifications are send - one for domain registration/transfer and one more - independent on domain operation. But only one notification is expected to be sent.
In PBA 5.4 there was possibility to set one notification template to be sent for any new subscription created on service plan. Since PBA 5.5 such option is enabled in generic service plans only ("Notification About Service Provisioning").
However if it was set to some value on domain service plan back in PBA 5.4 - corresponding notification (additionally to domain registration/transfer) will be send for any new subscription based on such service plan.
Since PBA 5.5. such option is remove from domain service plans and corresponding value can not be seen or modified from control panel. But it is left in PBA database. Please, contact Parallels Technical Support to remove unneeded template from database.