overdue: fix bug where add-ons are cancelled multiple times

Overdue was cancelling add-ons but entitlement is doing the same.
This was leading to duplicate CANCEL events in subscription_events for
add-ons.

This bug is actually quite old. We did fix it by
12d21a5fefb3708ba9feeadb5116e1b211f7a653 but we re-introduced it with
4bdae521d107ef04adaef9ff04162778ee91f35e.

This should hopefully fix #94.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>