They probably should have mentioned that Duo is a name to begin with. How do I know what a duo might be. (Yes, not putting it at the start of the sentence would have helped as well.)
(Not sure why everyone is going into Hänsel-und-Gretel mode here, but then, Esperanto is about meeting other cultures.)
While normally it would be, regard the 16th rule of Esperanto:
> The final vowel of the noun and the article may be dropped and replaced by an apostrophe (without effect on stress).
Because of this, it is perfectly correct Esperanto to leave off the -o, but technically it should be sankta fek' with an apostrophe.