"Do you dare to do it?"
Translation:Tør du gjøre det?
Duo accepted the 'å gjøre' form without telling me there is a typo in my sentence. Is that really correct or a bug?
Yes, that's correct. Keeping the infinitive marker is considered optional after the verb "å tørre/tore" when it's used in the meaning of "to dare".
In certain phrases, like "om jeg tør spørre" ("if I may ask"), it's acting as a strict modal and cannot be followed by an infinitive marker.
Other verbs that behave similarly: trenge, behøve, gidde, makte, orke, våge