Renewals self billing commission doubled the commission (renewal completed by admin offline and then paid)
I have tracked down the problem - it is not the renewals, it is the coupon code discount & partner discount combination.
If you use a coupon code, the discount amount is being added to the partner commission (or discount if partner is paying) as well as being its own discount line.
If you add a discount code and select the partner is paying you will see the issue.
Warranty that has expired with a partner. From Admin > Renewals > Completed the renewal as offline payment Ticked to say invoice paid. At this point the SB commission to partner had double.