How does graduate admission committee detect plagiarism in letter of recommendation? If my current recommender recycles the letter from previous recommender after each other's consent, will this be classified as plagiarism?

This is for MS programs to US universities FYI.
This will not be classified as plagiarism. A letter of recommendation does not pretend to be original, it is just a bureaucratic document that is supposed to give information on the extent to which the recommender recommends you.