There's two ways to watch Acorn on a Roku device and probably three ways to subscribe and they're all mutually exclusive. Are you trying to watch "Acorn on The Roku Channel" or using Acorn's stand-alone app? It's possible that you have signed up for both. Check your subscriptions at https://my.roku.com and if you have two for Acorn, cancel one of them. I don't know if one will indicate that you're on a free trial. If you want to try to avoid all of the subscription nonsense, cancel any Acorn subscription that shows there and subscribe directly on Acorn's website.
As I read your post again, it's not clear if you're trying to watch both on Acorn's website and on a Roku device. A subscription through Roku (that shows up at https://my.roku.com) may not work on Acorn's website. Most subscriptions through Roku come with the restriction that they only work on Roku devices. If you got that free trial on Acorn's website, then yes you'll be charged twice - once by Roku and once by Acorn.
As you can see, subscriptions are a real mess and most here would advise to never subscribe to anything through a third party like Roku, Amazon, Apple, etc.
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