In Puranas, there is a mention of Yama and Yami, who were twins. What does Hindu philosophy/scriptures say about twin(or triplet, quadruplet, e.t.c,) births for humans?
Garbha Upanishat gives a reason for birth of twins.
vyākulitamanaso'ndhāḥ khañjāḥ kubjā vāmanā bhavanti
anyonya vāyuparipīḍita śukradvaidhyādvidhā tanuḥ syāttato yugmāḥ prajāyante
If [at the time of impregnation] the parents are agitated, the child will be blind, crippled, hunch-backed or stunted. If the vital air moves around, the seed enters in two parts, resulting in twins.