It is said that there is a difference with time and the etheric, but not 'that' different- the truth is that the dimension of time is dependent on space, and in the etheric space may be something different than in the physical. So there may be a chance that you did go into the future- it is not unheard of.
However, since you were somewhat napping when you saw it, I could classify this as a possible hypnopomp, or possibly a feedback effect- you see, when you sleep you expand your energy body (which is why we say you project every time you sleep) and if you woke before reintegrating, it's a chance you saw your projected double before reintegration.
There is also a different neurological phenomenon called autoscopy, which happens to epileptics. But I don't know how common it is.