After migrating from 6 to 7 (Version 7.0-20131031-002505), there is a monthly concurrency with the invoice scheduler.

I consider it a concurrency issue, because of the erratic results: Some months there are a few double invoices, somtimes there are many double invoices.

So, I could either write something, that checks for doubles and deletes them or figure out the redundant source of redundancy.

How do I debug such a thing? Did you have similar experiences?

I searched the forum but have not yet found some fitting debuging approach. I will have a dump of the db and the sourcecode at hand.

Update: This is the module [0] on github. Odony added a FIXME in line 94 discussing locks. Is this related?

[0]: https://github.com/odoo/odoo/tree/9d9e3d2ef84eb5cc5a5ef4eec11127d17164e485/addons/subscription