Original poster here. As for the questions and suggestions so far: No, I haven't had a card failure on location, touch wood. Probably I would simply misplace a card or drop it down a sewer grate first. That's why I want to duplicate cards while I have the opportunity.
Regarding dual-slot cameras: Yes, they would do the trick. I'm just not ready to replace two camera bodies for this one purpose.
As many people point out, flash memory is very cheap, so I figure somebody must make a simple device to copy files from one card to another. No?
MB