Translation:While I was talking to you, I understood Duo.
I would say yes, it is incorrect.
Firstly, get is more colloquial than understand, and secondly, it's probably most acceptable in the collocation get it = understand the meaning of the current conversation, than in get someone for "understand what that person is saying".
There is also Gotcha which does come from "I got you" = I have understood you, but I probably wouldn't write that unless I'm being very informal.
So "I get it" or "I don't get it" or "What I don't get is how ...." are OK, I would say, but I would probably not accept other uses of "get" for "understand" if I were Duolingo.