Replacing a light switch that has 2 red wires and 1 black- one red was on the screw the other red was back stabbing the same portion
The simple answer is to connect your new switch in an electrically equivalent way -- pigtail the two reds together and attach it to the screw.  This will guarantee the switch functions exactly as it does now.

If instead you want to disconnect one of the red wires, you need to figure out where they go.  It's possible that they're actually the hot wires  and the black is the switched hot, and one of the reds continues on to power other devices.  In that case, you should not disconnect either of them.  Or it's possible the reds are the switched-hot wires, and that one of them is no longer in use, and could be capped off, but you'll need to figure out which is which.