von is commonly used in modern German to express possession, and is gaining ground over the genitive. (Perhaps the genitive will eventually disappear completely.)
der beste Freund Duos (with genitive) would also be possible, but der beste Freund von Duo sounds a bit more natural to me.
Weak adjective inflection after the definite article der.
When there's a definite article, the gender, number, and case of the noun are shown by the article, so the adjective ending after a definite article is just -e or -en without regard for gender, number, or case -- in the nominative singular, it's -e for all genders.
der beste Mann, die beste Frau, das beste Kind.
It's good English but you have changed the word order of the German sentence. That was a bit too creative.
Der beste Freund von Duo ist eine Ente. Word for word translation: The best friend of Duo is a duck. Does it work? Yes. Does it sound normal? Yes, but...
Duo's best friend is a duck. This sound's better.