Whatever you see there is what Roku is billing you.
If you only see the item you expect there, then the other is not a billing from Roku. Rather, a scammer has your payment card information and has submitted a charge, putting "Roku" in the description. You see, the description is a free text field that can contain anything, and scammers know how to put something that looks legit there.
If that's what is going on, print out your subscriptions page information as proof of what your did and didn't authorize, and submit a fraud claim to your bank. Sooner rather than later, as the scammer could shut down and move along any time.
Also, go to https://support.roku.com/contactus and submit a billing issue. As I said, Roku didn't do it, but do everything that must be done in order to get this resolved and hopefully get your money back.
DBDukes Roku Community Streaming Expert Note: I am not a Roku employee.
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