This means that for your son's service on 8/27, the hospital sent in duplicate claims for payments.  Your insurance company paid the claim the first time, so it denied the second claim, which is reasonable. 

My guess is that the second duplicate claim was sent after the insurance company sent the payment but before the hospital received it.  Keep the first explanation of benefits, which shows that insurance company already made payment.  Ignore the second one for now.  Usually the hospital's record will adjust to show payment. 

"Duplicate of charges previously considered on claim #*****" is the insurance company's reason for denying the claim.