I infer that your alternators are dying electrically, not mechanically due to bearing failure or some such.
It's not quite clear to me how a battery can cause an alternator to fail and still test out OK, but I'd sure suspect that the battery might be the culprit because it is about the only thing likely to slam a big enough load on the alternator to damage it. Furthermore, if the battery is working properly it's not that easy for anything else in the car to damage the alternator without blowing fuses or bursting into flames.
Maybe someone can suggest a failure mechanism and or diagnostics.
In any case, batteries are not built to last forever, and I infer that yours might be elderly because you didn't mention replacing it. Replacing the battery will be cheaper than buying yet another alternator. I don't usually favor throwing parts at problems, but in this case, I think if it were me, I might replace the battery as well as the alternator this time.