Hello RJG1999,
Welcome to Apple Support Communities. We understand that you already have an Apple Music subscription until July and would like to know what happens if you subscribed to Apple One. We'd be happy to get your question answered.
"If you already subscribe to Apple Music, Apple News+, Apple Arcade, or Apple TV+, you don't need to cancel those subscriptions. They will automatically be canceled when you're billed for Apple One"
Bundle Apple subscriptions with Apple One
With that being said, when you subscribe to Apple One subscription, you'll get a refund for the prorated period remains for your annual Apple Music subscription. This way you won't be charged twice for your Apple Music subscription. Also, as stated in the the link above, you don't need to cancel your Apple Music subscription as it will be cancelled automatically when you subscribe to Apple One.
We hope this information is useful. Kind regards.