Problem:
Picture #1 shows the drive plate on this alternator which has been scored by the fan.

Tjis is caused by the belt tensioner not operating properly.
The tensioner could be tight or possibly seized, which puts extreme tension on the drive end bearing, causing premature bearing failure.

This can also cause the rear bearing to fail.
( Refer to picture #2 )

Solution:

It is extremely important that the installer check the operation of the belt tensioner before installing any alternator.
If it does not work properly, REPLACE IT.