This is another one of my experiences that took place when I was 8. I was living in my childhood home (mentioned in other stories) and I shared a bedroom with my brother who was 5 or 6 at the time. We had a bunk bed and I had the bottom bunk while he took the top. I started having sleep problems around this age, and the experience I had happened on two separate nights.
The first night, I was laying in bed, trying to sink into sleep. I remember that I was awake, because I distinctly remember my dog laying in bed beside me. I turned over to where my back was facing him and while I was readjusting my pillow, my eyes caught something at the foot of my bed. When I looked, I saw my brother laying down at the end of my bed, with his head propped up on his elbow, looking at me blankly. I was confused, so I was going to ask him what was going on, because I thought maybe he had a nightmare or something. But I barely got his name out of my mouth before my brother disappeared. He didn't just zap away either, it was like he evaporated and faded away. I had no idea what just happened, but I eventually went back to sleep.
The second night, I was laying in my bed again, trying to sleep, but I had to go to the bathroom really bad and it couldn't wait until the next morning. I got up to make my way to the bathroom.
The way that our house was set up, my bedroom door and the bathroom door were directly across from each other, separated by a narrow hallway. Both doors were always left open, so as soon as I got to the doorway of my room, I could see into the bathroom, or at least I would have been able to if it wasn't dark.
When I got to the doorway, I could see into the bathroom (this was before I needed glasses) and even though it was very dark, I could clearly see my brother at the sink brushing his teeth. I thought this was really strange, so I said, "Jake, what are you doing?" He was surprised, so he looked over at me, and no sooner did he look at me, he evaporated and disappeared just like last time.
I was very confused and a little creeped out but not scared because at this time I still thought that this somehow really was my brother. I forgot about having to pee, so I went back to my room and saw my brother's form in bed. He was still asleep!
I didn't learn about dopplegangers until much later in my life, but this is a possibility to explain what happened. I have also heard that astral projection could explain this, but it seems unlikely. My brother wouldn't astral-anything because he has never believed in anything like that and wouldn't try it.