The word Doppelganger comes from German Folklore (Double Walker) is what the word literally means, and it refers to a paranormal double of a living person.
Here is where it gets a bit muddy….Some sources say it is an evil apparition that takes on your form (and the only entities that can do that are usually demonic in nature). Other sources say it is strictly an omen of death, if you see your Doppelganger it is the ‘grim reaper’ looking back at you, and will die shortly.
I have heard of instances however where people see themselves in past form (they see themselves in a residual haunting) meaning something that happens over and over with or without anyone noticing. And that is ALWAYS attached to a horrific event in the past. This however is not a TRUE Doppelganger, because you are not seeing yourself here and now.
The only way they have enough energy to move across the room, or turn on the faucet is if the are a very in-tune sprit, or if they are demonic in nature. A really strong ghost can move things (small things) turn on lights, etc. Only a demonic spirit can move heavy objects and hurt someone.
This is supposedly done by the manipulation of energy around the spirit. They take the energy (and negative energy is very strong) and use it to manipulate things around us.