When a premature baby develops Necrotizing Entercolitis for the second time is that concidered medical negligence?

I am sorry that your newborn is experiencing such difficulties.  But in a brief search for causes of NEC, it does not appear that factors under the control of the hospital would influence the occurrence of NEC, see http://necsociety.org/known-risk-factors/.  If you wish to be certain, you should have your baby's medical records reviewed by a medical expert in the field of neonatal care - but this can be very costly, running into several thousands of dollars to obtain an expert opinion.  Unless you have information from a care provider that the cause of the NEC was the responsibility of the hospital, it may not make sense to invest the money in an expert to be told that it was simply very bad luck.