Do surrogates get paid double if they find out they're carrying twins?

No, they do not get paid double, but there is usually an additional fee, around $5,000.

As for your second question, the only parents who would agree to work with a surrogate who is only willing to carry one baby are parents who adamantly only want one baby, and that's very unlikely seeing as how expensive it is, many parents would love it if twins came so they can get two babies with one cycle. Through IVF, the clinics typically put 2-3 embryos in, as long as there are that many available. The odds are good (not great) of there being more than one baby, so if the surrogate is not willing to carry more than one, that needs to be made clear up front to the parents and you guys would need to agree to only transfer one embryo. Legally, the parents can't really stop the surrogate from aborting extra babies. However, the surrogate would be in breach of contract and would likely get sued by the parents.