1. To duplicate go to the end and use the command

/kill @e[type=EnderDragon]

You will notice the boss bar/health bar does not go away.
2. After a while the Ender Dragon will come flying out of nowhere, and the process can be repeated all over again.
It should also be noted that neither the portal, the dragon egg, the gateway, or the xp spawns.This is less of a concern, although other mobs, when killed with a command, while they do not drop xp, they do drop their items...

Killing the Ender Dragon seems to be fine if the player lands the last hit though. Use the command

/entitydata @e[type=EnderDragon] {HealF:0.5}

Next punch the dragon a few times to get it to do the death animation. The portal spawns, xp drops, the dragon egg appears, the boss bar goes away, and the gateway appears.