When they die, they don’t “change back.” One way to identify a dead doppelgänger is to cut it open: you’ll note that the placement of their organs is wrong and that their bones deviate slightly. The number of doppelgängers living among us humans remains unknown. We have never seen whole communities taken over. They prefer to spread, spending years in the wilderness, living as rangers, charcoal burners, wandering monks and hermits.
The entire magic school of body alteration is based on doppelgänger magic. If you befriend a doppelgänger, it might teach you disguise and shape change. Apparently, shape shift (4th circle) is beyond their ken. They are limited to human shapes. We don’t know of doppelgängers in non-human societies.
HD 4 AC 5 1d12 F10 MV 9 ML 10 XP 190; shape change at will
Numbers: 1d4. When an infestation starts spreading, there will be more than one.