Installed apps are tied to your Roku account, so if both Roku devices are linked to the same Roku account (check under Settings->System->About if you're not sure) then any app added/removed on one will be added/removed on the other. It could just be that you removed it and readded on one and it hasn't fully propagated to the other. Perform a "System restart" under Settings->System or Settings->System-Power (as appropriate) and/or a "Software update" under Settings->System to see if it clears up the problem.
By the way, when you remove and reinstall, make sure you do a "System restart" after the removal and before the reinstall or any information that the app has stored on the device won't be removed.
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