It's a text about swimming practice, in Philadelphia today. A young sportswoman (a college girl) says : "We have doubles today" (meaning during the practice). I've been searching all over Google, didn't find nothing. What could it mean, "doubles", here ? It's definitely not a "doublé", like one might say about a competition victory. Could it be a "relais" with two swimmers only (a "relais" is usually by teams of four). Or maybe a double training session ?