What the title said, I read many books and stories when they said in older times, multiples would bring about misfortune. This belief didnt exist in my country so I'm curious about whether this was true or not. And how did people came to believe so?
Some may answer this with authority here, but this is more of an ethnographic question and historical. You may want to also ask this question of /r/Askanthropology.
As to the question of "how did people come to believe this sort of thing": the roots of cultural beliefs are often buried in prehistory, so understanding how these beliefs can be extremely difficult to understand.