You bring up a good point about dying, the and getting hit. [Knock-back is a no]
I would say, if you'd die, it'd turn into a random white orb or something, and be completely useless. Getting hit would cause you both to lose that much HP, like normal doppelgangers.
Ah, double-edge sword.
YOU DIE 2 TIMES FASTER D<.
If anything, I rather equip the doppelganger like a sprite, and do double damage. Though you can receive double the hurt...
BUT WHAT IF YOU MAKE THE DOPPELGANGER TAKE THE DAMAGE INSTEAD 8O?!
And if the dopppleganger is not on me like a sprite, it's gonna be hard to control and you can take serious damage xD.