I came across another question under the same vertical. Well, as of now it more of a theory or just a concept. There is no medical proof, test or studies that exist to ascertain the same. But the concept revolves around polar twins.
It essentially involves the female sex cell - ova, undergoing a process of division called meiosis to reproduce. The resulting 4 granddaughter haploid cells have half a set of chromosomes. One of the four egg cells called oocyte is available for fertilization and the rest typically fade away called polar bodies. But as per the concept of polar body twinning, both the oocyte and polar body would somehow fertilize resulting in twins.
So in theory, polar twins share 75% of the genetical makers which is less than identical twins but more than fraternal twins. And they surely do look similar but not exactly alike. Alas! Its just a theory and that is one more mystery in my Sherlock Holmes life.