Lots of cheap poor quality remanufactured alternators being sold nowadays. Unless you have something added onto your car that would make your alternator work harder than its designed too, my first guess would go to the quality of the replacement part that you have been using.
Before putting another alternator on Make sure the battery is completely charged. Modern alternators are made more for maintaining rather than 'recharging' a battery , therefore a dead battery can kill a new alt.Also check harness for proper signal voltage.(ford info)