Considering the Doppelganger is a role that can burn up its uses quickly, this seems to make the most sense. Dop uses its Doc save 1st round, Investigation 2nd round and VK 3rd round so long as the real doctor can stay hidden... seems like it could really put the TP in a good situation.. unless, of course, the Lackey blocks the Dop's save and the SK and Mafia both kill them.