Possible duplicate billing for some TV streaming services

Apple won’t know where and for what you subscribed to elsewhere, but all your subscriptions through Apple are listed in your subscriptions overview.

See your subscriptions overview - Apple.

All the content and subscriptions that you have paid for, are also specified in your purchase history. View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support

Some content providers, e.g. Paramount+, offer their services through their own app and also as channel inside the Apple TV app. These types of subscriptions have different terms and conditions, different perks and limitations, and are not exchangeable as such. If the intended link for the correct type is mismatched somehow, then it could alert for a missing active subscription. See the active setting for preferred content provider under the How to Watch header on the Details page for any show in the Apple TV app.

Not sure about Discovery+. That seems to be a singular unambiguous streaming service at this time. In a few months, Discovery+ and HBO Max may make some changes, and that may or may not affect Discovery+ as a stand-alone service.