Pointing out previous postings is not to cut off further questions. It's just to point to previous answers to get you started. You can search the AskHistorians FAQ with a Google query like

twin inurl:reddit.com/r/AskHistorians/wiki/faq

(I find the built-in Reddit search to be far less useful, missing lots of hits, and I think it doesn't search the FAQ.)

The two FAQ items currently are

• How were twins treated under feudal inheritance law?, /u/snackburros and /u/HabseligkeitDerLiebe
• Has the birth of twins ever complicated lines of succession?, /u/constanto and BarnabusStinsonIV. (I'm stopping u-references because I'm told that, if you have more than three in one post, it doesn't ping any of them.)

If you do a Reddit built-in search for

twin

you'll get 20 hits for the question. I'm afraid I haven't bothered to look into them, except where I answered the same question 4 months ago, where I added

• How were firstborn male twins perceived in Medieval times?, with Kjell_Aronsen on the famed Beaumonts of Norman England.

• How would the line of succession in royalty be affected by the birth of twins, triplets, etc? Would it always be the first one to come out? Were there exceptions? What about in something like a c-section, especially if the mother died giving birth?: larkspurblossom