Chonic Alternator Failure

When cold, the alternator will put out a higher voltage but even cold, 15.75v is too high; something is messing with the regulation system. The battery seems to be Ok if it is at 12.6v after sitting, so focus on the alternator and VR.

What is the battery voltage with the system cold and running, measured with the VM leads direct on the battery terminals? Also, what is the direct alternator output at a very high idle (like 1500 rpm), and cold? It is important to measure and share that information.

BTW, you're not losing fields, you are blowing diodes in these alternators. The described symptoms are classic for that.

It sounds like you have 2 problems:
1) Lack of voltage regulation since the alternator output is too high
2) It is very likely that there is some heavy, or spiking load in the car that blows diodes one by one, and that is not showing up on the ammeter. There are a number of circuits that take a feed direct from the alternator output and don't get fed via the ammeter when the car is running; they only show a stready ammeter draw when the car is not running. Examples: horn, headlights, some of the turn signal stuff, etc. Any spiking shorts in these or the associated wiring and switches will load the alternator and you won't necessarily see it spike on the ammeter.

You need to run this down rather than just swapping the parts. It is nice that the lights and such don't dim, but this is likley overcharging the battery which will eventually fail, and it could be a precourser to worse. Some light dimming with a correctly running system at low engine speeds is better than a chronic or worse problem.

And, just IMO, so take it as just that, dump the Duralast stuff and go with something better. You at least got your money's worth in replacement parts!