A doppelganger has long been believed to be some manner of shapeshifter, and to a limited degree this is true. A doppelganger is a parasitic creature that lives off of the life force generated by other living creatures. A doppelganger subsists in this way by engulfing a living creature and then rendering its gelatinous form nearly invisible. The doppelganger in this way takes on the appearance of the creature it has consumed, and begins to search for a new host for when the life force of the current creature has been consumed. Doppelgangers are psionic creatures, and are able to scan the recent memories of their prey so that they can better assume the role of the target. This leaves them somewhat vulnerable in that they are unable to recall information that precedes a certain time in the recent past, but doppelgangers are also frequently skilled liars, which allows them to generally escape without getting noticed.
In most cases, it takes a doppelganger 5 to 6 weeks to completely devour its prey. After this point, it may continue to take on the appearance of its prey for another 5 to 6 weeks until it finally loses hold of the prey's spiritual energy. If the prey has somehow not been freed by this point, its chances of resurrection are substantially reduced.
Generally a doppelganger can only be revealed through careful investigation and observation. It is said that doppelgangers leave behind a strange, gooey residue - but even this can be hard to track or even detect. They can be detected through scrying or other forms of magical detection, but many doppelgangers purchase or learn ways to avoid such detection.