I would just keep them all. They aren't that expensive so you wouldn't be getting a ton of money back (or store credit). The one we had got wet and stopped working so I had to go out and buy one but it would have been great to have a backup. I ended up buying the 3 in 1 thermometers.
Agreed with pp's - rectal is most accurate. My first thought was - I do NOT want to take a temperature in my baby's bum, but it's really not a big deal after you do it once.