When a user subscribes for a plan and pays the subscription fee, he should not be able to subscribe for the same plan in that same period, again. Maybe it's possible to gray out the subscribe button until the next due payment.
As to Paid Levels mod, subscription option should be displayed to the member level only, that's valid for level upgrade.
Dev-Team, could you please consider my proposal?
Edit: I attached a screenshot for an example. I'm premium member on a monthly subscription plan. Why do I then still see the upgrade button?