What is Doppelganger's attack when in the HQ?
Doppelganger's attack would then be 0.

The rules insert for Paint the Town Red explicitly covers this case under the Card Clarification for the Invade the Daily Bugle HQ scheme:


  If there is a Villain in the HQ space under Doppelganger, or if Doppelganger is in the HQ, then Doppelganger's [attack] is 0.