If your customer purchased a product on another system (like your own Web site), we won't be able to do anything.
You should remove the purchase buttons and inhibit the presentation of paywalls.
For every purchase made on mobile with Purchasely we protect you from unwanted double purchase.
If you attempt to purchase the same plan (monthly) of the same product on the same platform (App Store, Play Store …) the store will notify you that you already purchased and you won't be charged.
If you try to purchase a different plan of the same product on the same platform, you will be upgraded or downgraded to the new plan as long as your products are in the same Subscription group.
Subscription groups don't exist on the Google Play Store but Purchasely SDK will be cancelling the previous one and starting the new one respecting the migration policies.
If you try to purchase a plan of the same product on a different platform our SDK will protect unwanted double purchase by displaying an alert to validate the purchase. If the user validates, he will have the same subscription twice so when the purchase is validated we explain him how to cancel the subscription.