I'm being charged twice for one subscription for Amazon Music

Check you haven't subscribed to the same service twice, once through Apple and a second time directly with the service provider. E.g. : Join Apple Music on your iPhone, iPad, iPod touch, Mac, or PC - https://support.apple.com/HT204930 - "Make sure to join Apple Music through either Apple or your mobile phone service provider, so that you're not charged by both."

and

Get help with subscriptions not billed through Apple - https://support.apple.com/HT207865

To ask about a subscription:

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions - https://www.apple.com/emea/support/itunes/contact.html

or

Worldwide iTunes support - https://support.apple.com/country-selector/itunes