Some of our users sometimes experience an unfortunate occurrence of their credit card being charged twice for the Premium Membership. In 99% of all cases, the reason for this is that the user did not Sign In the account they have purchased the subscription with.
iOS Subscriptions are separated in several groups due to Apple's policy, allowing the user to purchase Premium multiple times. For example, the 59.99 USD one and 29.99 USD (50% discount) one are located in different groups and thus can be purchased separately. The user creates an account and purchases the 59.99 USD one and then creates a different account by mistake and, seeing there is no Premium, purchases the 29.99 USD one (when there is an ongoing sale).
This also occurs when the user gets a new phone and does not back up the data on the website. In this case there would be no way to Sign In the old account in the first place. For more information about this, please check the following article.
We sincerely apologize if this is something that happened to you and we are working on creating a solution or at least adding more preventive measures within the app.
Please check the following article on how to cancel and refund an iOS Subscription.