Double PayPal recurring subscriptions

Is it normal for s2Member to accept a new recurring subscription for an existing subscriber who already has a non-expired recurring subscription?

Details:
– Our site uses standard PayPal, not Pro, no special forms.
– We offer recurring subscriptions via a simple button transaction.
– For the case in point, a subscriber has seen double PayPal payments for several months.
– as an aside, we have not had IPN/RTN logging enabled during this time

This is what I speculate happened:
– The subscriber joined in Feb 2013 and made several monthly payments under a Profile ID of I-VB1….
– In May, the subscriber asked how to change credit cards and we gave instructions for how to do that on PayPal.
– Since then, a new Profile ID, I-BLJ… has been associated with that subscriber’s payments. He just now told us that he’s been paying twice as much at PayPal … although our payment notifications (via s2Member API from the WordPress site) are showing only the later Profile ID, I-BLJ…
– A check of WordPress users, at the time of the complaint, shows only one user with the appropriate identity, and the Profile ID of I-BLJ…
– We refunded that recurring payment transaction via PayPal, which generated a cancellation transaction.
– The WordPress user record has now reverted to the original Profile ID, I-VB1…

From this, I’m guessing that the subscriber resubscribed after changing his credit card information on PayPal, and that s2Member happily accepted the subscription.

Is this normal? Wouldn’t it make sense to not accept the new subscription if a non-expired subscription still exists?

S2Member should not be able to do that. As far as I know, it cannot handle two EOT or 2 concurrent recurring membership payments.

That being said, it made me realized that I had one customer who has been paying twice, every month. I’ll be following this thread for an explanation. Maybe a bug in the system can allow us to offer two concurrent membership payments.