Were Yaakov and Esav monozygotic ("identical") twins, or dizygotic ("fraternal") twins?
(Inspired by Sabbahillel's answer to Why is the birth of twins a Chidush for Rivka?)
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Chizkuni (Bereishis 25:25), quoting Rabi Yosi, says that that Esav was conceived by the last drop, and Yaakov by the first drop, and first in is last out. Thus, according to them, they were clearly dizygotic twins, as both eggs must have been fertilized seperately.
Ibn Ezra there (same link) says that there were two שליות (amniotic sacs or placenta?) that broke at once, also implying that they were dizygotic twins.
Rav Hirsch (quoted here) claims that they were monozygotic twins, and this was the detail of the pregnancy that shocked Rivkah.
Various modern medical professionals (see here, for example) have concluded that Yaakov and Esav were monozygotic twins, and that this was the first documented case of TTTS. There are various indications from the Parshiyos that indicate this, such as the descriptions of Yaakov and Esav, Yaakov holding Esav's heel etc.