I am not sure if doppleganger is what what we have experienced, or what is going on.  This has happened multiple times, to more than one member of my family.

The first time, my mom and I both saw the same thing. I was babysitting my great niece, and she (age 4) had been running around, and being a normal, energetic child. I was frustrated with her running in circles in the house, and when she darted through the doorway AGAIN, I yelled "Madison! Get back in here and settle down!"  From behind me, I hear my daughter say hesitantly " Um, Mom?"  I turned and looked and Madison was sitting on my daughters lap. My daughter and son were looking at me like I was nuts. I looked at my mom and she had a weird look on her face. I said "You saw her too, didnt you?"  She confirmed it, and we discussed what we had seen. My niece. Going through the door. Her red shirt, her blonde hair, at the right level for her height.... I don't know WHAT we saw, but it was not her.

It was more than a year later the next time something like that was seen. My brother, who is older than I am, was in my mothers kitchen when I got there. he was freaking out. he explained that he had seen HIMSELF walking across the back yard...through the window. wearing the same clothes he was in. (simple, right? just a reflection? He is 51, and the "figure" was walking in the opposite direction)

Upon discussion, my mom revealed that she has seen both my brother and myself when there is no way we were there, nor was she sleeping.

I have not seen MYself. Nor has my mom seen HERself.  BUT...we have seen odd things. (yes, more than just this, but that is another post)
I should mention that one of the other things we have seen has been shadow people.

Any ideas? theories? thoughts? Please share

I have never heard anything positive about doppelgangers save some positive remarks made about them by those preaching dreamy eyed non-demon-infested new age crap. In my opinion, doppelgangers are negative entities that assume your form or they are your dark spiritual twin. In either case, they would not come unless something bad were to follow because they do not visit you from the kindness of their heart, but out of the spirit of malicious taunting.

Shadow people wouldn't have anything to do with it unless they are "volkrotielnychs." Very odd, though. I wish I could help you more, but whatever causes this is probably harmless.