Just because you have one device doesn’t mean that you have accidentally created more than one account. Check this link to see for other accounts using your payment details.
Are you 100% certain you’re paying twice for boost? Is it not a discounted cinema or entertainment membership that you’re also paying for? What does your account up coming payments show?
If you are indeed paying twice for boost and or also somehow got another account jump onto live chat to speak with staff. They should be able to sort you out, make sure to click “chat online” within the green box, don’t hit “get help quicker” as this is a bot.
At the same time for any duplicate accounts send an email to support to ask for it to be closed. See below picture for the email address.
FYI that I do not work for NOW, just a NOW customer trying to help