Alternator going bad twice after five months

There are a number of things that can cause premature alternator failure.

1. Substandard replacement parts - it could just be that you have gotten substandard parts that fail more readily
2. Poor/damaged ground - a bad ground can cause the alternator to surge and damage the voltage regulator
3. Weak/bad battery - a battery that is not holding a charge becomes a load on the system and can cause the alternator to overwork (I know you said you were told the battery is not the problem, but I would check it again)
4. Aftermarket accessories - aftermarket accessories can put a huge load on the alternator causing it to work itself to death; things like audio amplifiers, extra lighting, etc. can create extra drain on the electrical system
5. Belt slippage - a loose or slipping belt can cause the alternator to not function properly. This can result in dead battery, damaged alternator, or both

I had a 2000 Chevy Blazer where the secondary air pump had a short in it causing the alternator to fail 3 times in six months. There was no other indication in the car that was causing the issue: no check engine light no blown fuses. The secondary air pump was disconnected temporarily until a new one was put in. No more alternator problems.

Are you buying the same brand alternator? Try using one from different manufacturer.

Heat is bad for the diodes so if the alternator is anywhere near an exhaust manifold they do not have long life. I had an 85 300ZX with no turbo ;Because the turbo space was available ,Nissan put the alternator in the space. I think it was 3 alternators in 5 years.