I'd blame the cheap aftermarket tensioner or install before I'd blame the belt.
As far as I can tell, belts usually outlast the engine. I only replace mine on the Alero becaus eit has the original on it past 10 years AND it was making noise, AND there was a part out local to me. On my Escort I only replaced mine because I had a problem and got power steering fluid and antifreeze all over my serpentine belt.
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