Contact's second membership fails to inherit when created in back end using price set

Summary

Issue occurs when there are two membership types, each having the same relationship type configured, with different membership organisations so that a contact can have memberships of both types.

If a contact has memberships of both types and a relationship of the relevant type, the related contact should inherit both memberships. However, when creating memberships in the back end using a price set, this does not happen and the related contact only inherits one of the memberships.

The symptoms are the same as issue CRM-19735, which affects 4.6, except that in 4.7 it only occurs for memberships created in the back end using a price set. This is believed to be because CRM-17723 fixed it in 4.7 for online contribution pages.

Steps to replicate

1. Create membership type "Org Main Membership", with fee, 1 year fixed, Relationship Type "Employer of"
2. Create membership type "Org Additional Membership" with different member org, with fee, 1 year rolling, Relationship Type "Employer of"
3. Create membership price set "Org memberships" with field:
   Membership: CheckBox, with options:
   membership type: Org Main Membership
   membership type: Org Additional Membership
4. Create memberships for an org that has an employee (E.g. for Berlin Environmental Initiative in Civi test data set), via back end, using price set:
   Add Membership -> Choose price set -> Org memberships
   Select Org Main Membership + Org Additional Membership, tick Record Membership Payment?

Expected outcome

An employee of the org should inherit both memberships.

Actual outcome

The employee only inherits one of the memberships.

Affected Versions

Affects current dmaster, also affected dmaster when tested several weeks ago.

Does not affect 4.6, as CRM-19735 fixed it there for both online contribution pages and back end via a price set.

Fix

The fix https://github.com/civicrm/civicrm-core/pull/9520 for CRM-19735 (4.6) also fixes this issue in 4.7 .