If you noticed it within 24 hours and the flight date was more than 7 days out then you were definitely entitled to a full refund as opposed to a travel credit. You shouldn't even have needed to call them. You should have been able to cancel directly on the website for a full refund.

If booking less than 7 days out and/or if you waited more than 24 hours to cancel, then a travel credit would make sense. That said, JetBlue does have duplicate reservation checking so it should not have even been possible to book a second reservation for the same flights and passengers. One of those reservations would have immediately been canceled by their system (we can't remember if it's the original one or the new one but it's happened to us more than once).