"Are they twins?" - when you see a mum with two babies, approximately the same size and appear to be the same age you can be pretty sure they are twins. The correct question would be "how old are they?" that way if they are twins mum will answer "10 months" if they are not she will tell you their individual ages instead, and using your powers of deduction you can figure out if they are twins.
"Did you have help?" - as in reproductive help. This is not a question you should ask a stranger, or a family member for that matter. If you are stupid enough to ask this question don't be surprised if the mum replies "do you use pads or tampons?", it's just as personal and rude.
"Are they identical?" - this is only stupid if you have already been told they are opposite sex. For the record the word identical means exactly the same - if they are of opposite sex they are not exactly the same. But if you are stupid enough to ask this don't be surprised if mum replies "no, he has a penis".
"Double Trouble!" - OK this isn't a question, but still something you shouldn't say. First of all although yes it's "trouble" to have two babies at once, most of us are very happy with them - especially if the mum would have answered "yes" to the "did you have help" question. Second when the kids are old enough to understand do you really think they are going to constantly hear that they are "trouble"? Oh and another thing - you're not clever, we've heard it a million times!
Can you tell I ventured out with them yesterday :)