You should never fail at something crucial when it would cost you under $10 to ensure you succeed.
If you ever miss appointments that are really crucially important to you, that means you need multiple alarms.
If you have just one alarm, you might accidentally set it to "PM" instead of "AM," you might otherwise set it to the wrong hour, or your alarm could run out of batteries or the power could go off.
But, two alarms?
Much less likely.
When things go wrong, people tend to say "oh, I'm so stupid."
No, you're not stupid. Everyone has an alarm fail to go off at some point.
What is stupid is judging yourself and thinking it's a moral failing instead of solving the problem as a broken system.
Relying on one alarm? You're going to miss some crucial appointments.
Two alarms? Much less likely.
Three? Now you're talking. If it's important enough.