Double Tap is a hidden achievement/trophy in Call of Duty: Advanced Warfare in Exo Zombies. It requires the player to defeat Oz for the third and final time in the map Descent.
In order to do this, players have to complete the Reunion easter egg to unlock Double Feature mode for the map. Then, the players progress through the map until they kill Oz in his "fused" state. This is the secondary story easter egg achievement.