upgrade cancelling plans

i am using the new GPL cbsubs with newest cb.
i had a client upgrade from one plan to a higher on the 8th. he was then charged again on the 10th for his previous plan which he upgraded from. so even tho he upgraded from the plan he was still charged for both. why is this? and how can i fix that? i have had this happen a few times in the past.
thanks

Sounds like you're allowing multiple subscriptions to the same plan. Within your plans on the workflows tab scroll to the bottom and see if you're allowing multiple subscriptions.

all plans offered are set to "Only one subscription at a time"
is there any other reason that could cause this? im hoping this is
just a fluke because it really hurts my refund rate!
thanks

What payment gateway was used? Are you using recurring subscriptions? If the previous plan is a parent of the new plan then both plans will be subscribed to at same time (they need to be).

i use paypal. both plans are set to "top" for parent plan so they neither depend on the other. they are both independent plans:)
any other ideas?
thank you for the help

Are both single or recurring subscriptions? If recurring then for whatever reason it didn't cancel the recurring status of the subscription at PayPal. Ensure you've your PayPal gateway configured with API credentials so cancellation can be done from onsite.