As far as I know, you can't. Each mark refers to a single location; if you change this location with the
m command, the previous location is lost. There is no concept of a "mark value history" which can be accessed by the user.
Your best bet, if you want to avoid accidentally overwriting a mark, is to use a mark name which is hard to type accidentally on your keyboard. E.g. instead of using
mm (which could happen easily due to key chatter or double-tapping), use
mz (assuming a QWERTY layout).