When going through the Payment Details process, if you do it twice -- but not ever pay the first time -- the system says "This subscription is already active. By proceeding with the payment, you will extend its expiration date." It seems that the system is only checking to see if there's an internally-generated token (or whatever you want to call it), e.g. the "SUB-XXX" stuff you put into the memo, rather than if there is an _actual subscription_ or not.
