To unlock the Double Dose trophy in Sniper Elite V2 Remastered, you will need to kill two people with a single shot.
A good place to unlock this trophy is on the game's "Schoneberg Convoy" level. At the very start of the level, you will see a soldier stood leaning up against a wall. Stealth kill him, and the move to the truck in front and slight to the left of him.
A little further down the road, you will see two more soldiers. One is stood near a motorbike, with the second one patrolling near him. Wait for the patrolling soldier to return, and once they are lined up, press
to activate focus and headshot them.
You can see a video showing how to unlock the trophy below (credit to NTAGameGuides on YouTube):