“The definition of insanity is doing the same thing over and over again and expecting the results to be different. “  This is why condition of the alternator being removed from a vehicle

is one of the most important factors for successful alternator replacement.  Nothing in automotive repair is more irritating than doing the same job twice. 

Always closely inspect the alternator condition when removed from a vehicle. There are many things this inspection can tell you, and also tell you that the problem is destined to re occur.

Has the alternator been replaced before ? In other words, does the alternator look newer than the rest of the parts around it? Most alternator replacement would occur between 130-200K mileage. If this is premature, there is definitely suspicion of underlying cause. Most often the battery or aftermarket add ons.

The above image is an example of a vehicle with an oil leak. Another alternator should not be installed until the oil leak is fixed. The oil is drawn into the alternator by the air cooling fans inside the alternator. The detergents in the oil break down the brushes and the diodes. Brush decomposition would occur within days. 

You can also see pieces of shredded belt mixed in with the oil. The guy said he thinks the belt is OK. He is wrong. The belt would be oli soaked and a good percentage of it is missing because it’s in the alternator.