I'm not sure if everybody has a 'double/twin' but I believe there are many. I have actually met mine. Back in my teens I would be mistaken for another gal at many different social events. And, we had the same first name to make it more confusing to people. So in time I learned she was around my age and lived in the adjoining town to mine. And, she experienced the same the same with people thinking she was me. Well, one day I was taking the bus to the local popular teen swimming place and sitting directly across from me was her. We both immediately noticed each other as it was like looking in a mirror. So we got to chat some en route to swimming and learned how we were both experiencing the mistake in identities. Soon after my family moved to another town and I never saw her again. If it were today, with the social media, I'm sure we would be in touch. Strangely enough I had twin sons, fraternal, they don't look alike, just related. They just turned 42 last week.
While I don't believe we all have a "twin" that is identical to us...I do believe there are people who look enough like us that many might think they are our twin. I have read of some cases where women had twin babies (identical) and gave up one child because at the time they felt they could not afford two of them. Years later they end up finding their identical twin that they had never known about. I also read of a case where a married couple had identical twins and divorced while they were still babies. The couple decided to each take a child and neither one told the children they had a twin. Many years later when these twins somehow found each other yet they did not know they were even related until they had DNA tests done. So while in the above cases these people really do have an identical twin....most of us just have someone or maybe a few someones who look very much like us.