Access permissions are duplicated when pages are added to collections

[Kristina Hoeppner]

Noticed in Mahara 18.04

When pages already have access permissions, they are applied to the collection when the pages are added to the collection.

Normally, that should result in a joining of the permissions, but people / groups are listed multiple times.

To replicate:

1. Create 2 pages and share each with "Registered users".
2. Create a collection and add both pages to the collection.

Expected result: The access permissions show "Registered users" only once.

Actual result: "Registered users" show up twice.

When a portfolio page has been shared with a user for a specific date and the second page has been shared for a different date then both dates should be retained. However, when there are not additional conditions for a selected user or group of people, their permissions should only show up once.