A Doppleganger in occult terminology is a "spirit double", or rather, a demon that looks exactly like someone. In German folklore supposedly there have been people who saw their Doppleganger a few days before they died. [Note to Yahoo answers, I have answered the question]
Here's what you should do next.....
Q. I think my house is haunted, for real. What should I do?
A. First, you eliminate the natural explanations. Is anyone in the house on drugs? Seriously, are they? I've known quite a few pot smokers who had a "ghost" in their house. Stop taking drugs, and the ghosts will go away. Does the ghost only seem to bother certain people? A child who wants attetion? An elderly person in the first stages of alzheimers? Schitzophrenics often live in "haunted houses" and talk to aliens too. Is there a source of running water nearby? A noisy aquarium pump turned out to be the "ghost" in one house I know of. If the occurences can't be eliminated by natural explanations, then you should call in a minister to bless the house. DO NOT call anyone with an occult or New Age background, because it won't do any good, and will probably make your situation much worse. Coventer's Prison in England had a problem with visitors being attacked by unseen forces in the late 1990's. They called in a spiritualist "minister" (i.e., a medium) to exorcise the place. The medium pronounced the place cleared, but of course it didn't work. He died a few days later, and the occurences were worse than ever. They finally closed the site to visitors, because of the unexplained marks and cuts visitors kept getting! I also know of someone who thought they had ghosts in their home and called in some Wiccans to do a "cleansing ritual". The situation only got worse. Don't get occultists involved. No Wiccans, Voodooists, Santerios, Canduras, "white witches", ceremonial magicians, Rosicrucians, Theosophists, Cabalists, spiritualists, psychics, "spiritual advisors", or anything like that. These people think they know a lot, they'll act like they do, but they don't (I'm not being mean, I'm being honest).
Get a Christian minister from a Bible believing church (no Mormons, Unitarians, JW's, Unity, or other cults). Chances are there is one close by. If they don't want to handle it for some reason, call someone else. Get out a phone book if you have to. It won't be hard to find someone who can help you.