Several thousand fans cheered as Benny Andersson, Bjorn Ulvaeus, Anni-Frid Lyngstad and Agnetha Faltskog walked down the red carpet in Stockholm.

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"The biggest problem I see with these clubs is that people participate in them without having made sure from a health perspective that it's clear sailing, " says Dr. David Frid, a cardiologist at the Cleveland Clinic.

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