The Ethereal Doppleganger is a 3.0 creature-as with all the monsters in MM2. You might need to convert its abilities to a 3.5 version before continuing. It might also have its CR lowered in the process.
Here is link to someone who did a savage progression for the monster: (It also includes the 3.5 update for the creature)
In my opinion, upon looking at the Ethereal Doppleganger, I find the monster utterly ridiculous. The powers it possesses are completely broken. If I was to recreate the creature, I would find powers that are similar in psionics or spells already in the rules.