Programmatically creating or importing content creates additional rows in tblNotificationSubscription

Description

Steps to reproduce:

1. Create a project.
2. Navigate to the Export UI in CMS admin view.
3. Select the root page. Select "sub items" and "export files that the pages link to."
4. Click Export.
5. Delete any page. 
6. Check the # of rows in tblNotificationSubscription

   select count(*) from tblNotificationSubscription
   

1. Navigate to import screen.
2. Select the file from the previous import. Use the "update existing items with matching ID" option.
3. Begin import.
4. Check the # of rows in tblNotificationSubscription.

Expected result:
No increase in rows.

Actual result:
Rows increase.