Fixed checkout bug allowing unintentional downgrade or re-purchase of a lifetime license

We got a report where a user, in an edge case, could accidentally re-purchase a lifetime license from our Checkout. They already had a lifetime license for a particular plan. Then they accidentally entered that license key for “upgrade” to the same plan and therefore got charged again.

In this iteration, we have fixed this behavior by simply blocking it from happening. The UI will also show a proper message to the user asking to choose a different number of licenses.