are duplicate passports illegal?

Legally issued passports are legal, by definition. I used to have two passports for travel purposes, mainly because in some countries you wouldn't want to show up with certain other countries' entry/exit stamps in your passport. And if you're a dual national, you can of course have passports from both of your countries of nationality, nothing 'illegal' about that, either. So no, duplicate passports (whatever you mean by that, exactly) are not categorically illegal.

Yes.

yes