Fix double creation of subscription line items

[aldesantis]

In some circumstances, it looks like subscription line items are duplicated when saving a subscription via the backend. Not sure when exactly this happens or why.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It might be closed if no further activity occurs. Thank you for your contributions.