The doppleganger is merely etheric projection. When I am worried about someone, they tell me they heard my voice but didn't see me, and some have smelled my perfume while alone in their home. A few have actually seen me, especially when I am sleeping. People who are involved in projection or are naturally good at it can have multiple accounts of "doppleganger" sightings.
Likewise I've seen what I like to call "desire phantoms". When someone wants to be somewhere they cannot or is anticipating what they are going to do (but aren't getting their fast enough) they tend to project themselves to it before their physical body can follow. For instance, a few minutes before my husband gets home from work, there are times my daughter, myself, or even my dog have seen him through the window entering our gate. (If its my dog he begins to wag his tail, howl, and act like a goofball as he always does only for my husband.) Moments later, no one enters the door and I realize what one of us saw wasn't him in the flesh. But I do know he's on his way home and is anticipating his arrival. Usually in less than 15min, he comes home.
The old superstition goes that when you see someone's doppleganger, that they are in danger, are/will become ill, or die. This has some credence, but it isn't the only time as I stated above. When someone is ill or going to die, they have an intense desire to see the people or places they love and their body is no longer grounding their energy as it normally would because it is getting ready to let go of the soul completely. I've experienced a before death doppleganger. It was much more like a full haunting every night for a week at a certain time the house would smell like my husband's grandfather (not a pleasant smell mind you) and we could hear the sound of his cane banging as well as his shuffled walk downstairs. I saw him a few times during these nights, just for a moment. He died about a week later. We had his cane in our coat closet at the time (he was in a nursing home and could no longer walk). We gave the cane to my mother in law after his death. We told her to bury him with it, but she wouldn't. Well...its hers to be haunted now. I can understand why it was considered "evil" when someone was ill or dying because they don't act right (not like themselves, very chaotic, creepy, or wild).
I have never seen myself or heard from anyone who has seen their own doppleganger. I would assume that such a thing would be a malicious spirit, not apart of you, and that they are in fact threatening you personally. Something about a spirit taking on a physical person's form seems entirely up to no good.