Be sure to bring healing breeze as your only self heal. Dopple won't use it for some reason. And like the others said, bring sac skills and awaken the blood but dont use them. Just hit him with your wand/staff, preferably max damage, and heal with breeze whenever you need to.
I breezed through doppleganger with my N/Mo last week. I used curses like Price of Failure, Enfeeble, Insidious Parasite and Spiteful Spirit. I never attacked him. I just kept hexes up on him and watched him kill himself. Went pretty quick. However, I acquired Spiteful Spirit with a Drok run so many necros probably don't have this skill when their fighting the doppelganger. This was by far the easiest time I had beating him between my warrior, monk, ranger and necro.
Seeing how you can't get SS until you hit Camp Rankor/Droks, you can't use it against the doppleganger (if you have it, you got a run to those cities).
Price of Failure
Incidious Parasite
Enfeeblement
Reversal of Fortune
Healing Breeze (I'm not sure if they corrected it or not; last I heard, doppleganger won't use it unless you use a health degen spell on it)
Rebirth
Ressurect
Signet of Capture
You can also do, as already suggested, get nothing but sac skills.