"Zelfs Duo spreekt Nederlands."
Translation:Even Duo speaks Dutch.
I think that the English "Duo himself speaks Dutch" has the same meaning as the given correct answer
Or Duo zelf spreekt Nederlands
There's a difference between zelf (himself, herself, itself, etc) and zelfs. You use zelfs when something is against (all) expectations, or if something is more "extreme":
- Zelfs hij gaat naar het feest – Even he goes to the party
- Hij verdient zelfs meer dan ik – He earns even more than I (do)