It was originally meant as a better JavaScript and it was. It failed when none of the other browsers expressed interest in supporting it. It languished for a while and then was taken up by the Flutter team. At the time Flutter took it up it was somewhere around the level of Java 8 in features but not quite on par. Since then it’s seen some massive improvements to the type system and language. It’s completely null sound, not just null safe like Kotlin. It recently got records/tuples and one of the more capable pattern matching syntaxes I’ve ever seen in a functional imperative hybrid language. The next stable version of dart will introduce a compiler macro system that is very promising. The syntax isn’t always the prettiest due to it trying to not totally break old code. I do think that it offers a wide range of modern language features that competes heavily with Swift and Kotlin in the mobile space.
I think the responder means that duplicate code is usually easy to refactor into single methods. Typically I see copy pasted code that is changed just a little bit. However much of a duplicated function can be broken into smaller functions and the redundant code removed in favor of calling into the functions. Often what is left then becomes easier to reason about and refactor accordingly. I love the PRs that I make which delete more code than I add but still manage to add functionality. It doesn’t happen often but it’s fun when it does.