Subscription charged for 2 x 12 month periods serially instead of backdating

There's a member who was manually added to the site with an unpaid sub. It was pending his login and payment to start his 12 month sub. He didn't login at all and the 12 month period passed. He subsequently logged in and paid but this started a 12 month sub and then another immediately afterwards giving him a 24 month sub period.

My understanding was that the sub would backdate the period to the first start date and then add on the current period preventing a gap.

It sounds complex but if you can understand this can you advise?

Screenshots of the plan settings and the member dates attached.

Thanks

What payment provider are you using? Were they billed twice? So if I understand correctly they registered and created a basket 12 months ago, but never paid it. That should result in the basket just expiring so there's nothing to extend. Sounds like the payment IPN may have processed twice.