I have dual US/UK citizenship, which must be a fairly common combination. I'm amazed at how often people will adamantly tell me information about this, which usually completely wrong. "It's illegal to have both". "You have to choose which one you want at 18". "You only get citizenship from the country you were born in." "You can have dual citizenship, but only vote in one country". etc.

These are certainly true sometimes in some cases for some countries, but they're far from blanket rules.