Like most United promos the deal only applies to newly purchased tickets. This one also is restricted to US residents and trips originating in the USA. Connecting itineraries are eligible but only the Dulles-Dubai sector gets the bonus. Also notable is that only full fare (B or higher) tickets get the 100% bonus miles. Lower fares only get 50% bonus. Open-jaw and government fares are also excluded from the earning promo.
Time to start tracking DXB fares and see if there isn’t a mileage run to be had there.