Double stroller

Any of you moms with more than one invest in a double stroller. Currently my (almost) 2 year old son loves the stroller when we go on long outings. He will be almost 2 and a half by the time baby sister is born. Do you think it would be worth getting a double stroller or do you think he would likely only require the stroller for a few more months after baby? They are so expensive and I don't want to get one if we are only going to use it for a couple months.