Each person should only have one sticker subscription at a time per album. However, since subscriptions are handled through the App Store or Play Store, there may be certain factors, such as renewal timings, that result in duplicate subscriptions. In such cases, in addition to being sent an email notification, a notification will be displayed in Settings > Sticker Subscription.
Please check the status of your subscriptions and cancel any that are unwanted. See Canceling a sticker subscription for how to cancel.