Our system has the same thing. I can see how this would be a useful feature across other devices/technology – If I sign in this way, do this, etc.

I wonder what you're supposed to when the guy holding the gun says "Give the alarm code. Good. Now give me the duress code."

You give him any code. Its not like he is going to use it. If there is a pattern like ChuckMcM's where its +1 then give then give that one. Of course in all probability this scenario will never happen and the xkcd wrench is used instead.

How likely are you to actually remember the code when you're under duress, though?

More than 0%, which makes it more useful than not.

You could make it an easy one. Say your normal code is 5829. The panic code should be something like 1111. You'll never key it in accidentally, and people have codes like that, so it won't tip off an assailant.

the thing is, it's not the worst thing in the world if a bad person guesses it.