My sister has 50% (on average) of the same DNA as me. If I impregnate her, our child will (on average) have 75% of the same DNA as me. If I impregnate the child, it will (on average) have 87.5% of the same DNA as me, and so on.
With approximately 15 years per generation to sexual maturity, and (stretching what's possible a bit here) assuming I live to 105, that's 7 generations possible. That kid would have over 99% of the same DNA as me (on average).
Is that enough to make him appear as an identical twin to me?
Has this ever been attempted?