Why is the word "PLUS" now displayed at the top of my page?
I've been using the free version of Duo since I joined. I recently noticed that, at the top of the page next to "duolingo," it says "PLUS" in blue with a white square around it. I'm not sure when that appeared, but it hasn't been very long. I haven't signed up for a pay version, at least, not to my knowledge. Is PLUS also free? I hope I'm not being billed for something I didn't opt into, at least not on purpose. Is this a new thing?
Plus isn't free, and I've never heard of it being given out free. I'd check your app store bill, if I were you, lest you bought it by mistake somehow (there is information about refunds here). If you're definitely not being billed for it, then it would apparently be a felicitous bug that saves you from having to look at advertisements.
If it's trying to bill you for something you didn't sign up to, notify your credit card company toot suite and block any payments.