What is the English level of the non native English speakers, who complete courses from English?
Do you find it a handicap and get frustrated?
Did you have to feel confident with your English abilities in order to take a course from English?
Can you understand English movies without subtitles?
Does it effect your learning negatively, if you compare it to learning from your native tongue?
I think my English level is more than C1, but less than C2. The only thing that frustrates me a bit is when I make silly mistakes systematically in English (like "this apples" instead of "these apples") and get my answer wrong for using the wrong word. Yes, I can understand English movies and videos without subs, but I have some problems understanding some accents...
It was way more frustrating when I tried to do the PT to DE tree, as the Portuguese here is the Brazilian variant, and so it was full of words I have never heard as a native European Portuguese speaker :-/
Of course learning other languages through English is a bit frustrating, especially when you make a mistake in English, not in the target language. Or when you face a feature present in both your native language and target language, but not in English. Or vice versa, when the feature is present in English only. But this is the only option we, speakers of minor languages, have.
I don't think it's a handicap, and for certain languages outside Duo it's a requirement (trying to find Finnish material for Slovak is a nightmare). Most of the frustration and negative effect if you want to call it that comes from English lacking certain words or features that both Finnish and my target language have. I can't really answer your second question as I felt confident with my English before I knew Duo existed and one reason for the confidence is my capability of understanding spoken English without problems.
Yes, I can watch films without subtitles. I can also read Shakespeare and Chaucer, so I guess my English is rather good. Learning from English does not bother me at all. Quite the opposite. It just helps me to form new connections between different nuggets of linguistic knowledge. I also do reverse trees. So far they have all been English trees, but I am planning to do the German-French and French-German trees at some point even though I am not fluent in either language. Getting out of my comfort zone is a good thing and it will help me construct more of those aforementioned connections. :)