How can one zener go down?

Fixing a Mr. Coffee maker It went out the first time because of the 24V zener that was factory installed. and since I did not have time to order parts I paralleled 2 12V zeners..

Well it went out again after maybe 1-2 months, so after a few months to finally getting around to it I went around checking and sure enough one of the zeners is showing voltage forward and reverse bias but the other attached is fine.

I thought both would of been knocked out?

What should I do (other then replace) to get a robust zener?

If you're replacing a 24V zener with 12V zeners, you should connect the 12V zeners in series, and not parallel. If connected in parallel, they act as a 12V zener.

Zeners often fail as a short-circuit, which is why you have one with measurable [low-voltage] forward and reverse conduction. Both don't have to be destroyed, and if you truly have connected them in parallel, then the remaining zener need never fail as the failed zener is shunting most of the current.

For a more robust zener, without just buying a more robust zener... you can use many smaller matched zeners, or a transistor buffered zener.

I would just order a larger wattage zener in the correct 24v.

Also, please be aware in many installations the zener needs to have long enough legs as they can form a very large part of its heatsinking. I've seen some where the legs were bent in little spiral coils of a few turns, to give good heatsinking without having the zener 30 mm off the PCB.

The coils in the legs I've seen have been used to provide strain relief in older point-to-point and PC boards and some circuits on tie-point lugs (terminal strips).

It's done for air cooling of diodes on normal through-hole PCBs too dougy.

Many of the power diode and zener diode datasheets have specs/charts for dissipation etc for different lead lengths in air.