It's the be fruitful and multiply. Which was ok back when infant mortality was sky high and we were dealing with the plague and famines and stuff. But with today's modern medicine in developed countries it's really not necessary. If you can ever find it, there's a great article by Gore Vidal in the 25th anniversary issue of Playboy called "Sex vs. Politics and Vice Versa". This is one of the points he talks about in an article that deals mainly with how certain parts of Judaio/Christian scriptures are highlighted at different points in history to improve the lives of the wealthy/powerful. It was so good in its truthes.
As far as dressing them alike? Eyeew. I wouldn't even do that to twins if I had them. I never saw that as "cute". It's kind of sick actually.
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I'm just an ordinary human after all.