XML parsing with wrong parameters

I am parsing an xml where my contatct information is being updated wrong sometimes. This is the code.

// Check which first and last name is not empty   
                if(String.isEmpty(first) && String.isEmpty(last)){
                dohmh.First_Name__c = first_name;
                dohmh.Last_Name__c = last_name;
                //system.debug('first_name ' + first_name + ', ' + 'last_name ' + last_name); 
                }else{
                dohmh.First_Name__c = first;
                dohmh.Last_Name__c = last; 
                //system.debug('first ' + first + ', ' + 'last ' + last); 
                }
                // End of Check which first and last name is not empty

One thing what I observed is most of the times the value is from th e pervious xml even if the contact infor is null.

Thank you,
Alekhya