2017 Mazda6 - Tensioner and belt replacement question

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'17, Mazda 6 GT
Just went to my dealer for the tire and oil change and it seems that my tensioner is leaking and belt is a bit cracked. While I am not worried about the belt I am curious about the tensioner. Did anybody replace this ? Is an aftermarket part enough ?
My dealer quoted me for the replacement of tensioner and belts ~610$ + taxes which seems a lot of money.
 
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