Is food free in Duronto sleeper?
Yes. Food is served for free inside all coaches of Duronto Express. The food charges are already included in your ticket expense. With what I remember from my last booking in a Sleeper Duronto Coach, I was asked the choice of Vegetarian or Non Vegetarian at the time of booking. In case you missed it, you could just let your coach attendant know about your choice.
Check this pdf (page 6-7) from Indian Railway official website for the menu in sleeper class.
IRCTC mentions "catering charges Rs 190"(checked on 9 Jan 2017) when you book ticket in sleeper class. So, I think, It should be free. also check this link (page 6) for authorative reference: http://www.indianrailways.gov.in/railwayboard/uploads/directorate/coaching/TAG_2014-15/MenuMeals.pdf