Below is my apex class method
Creditwriteup is the custom object  into which i want attachment to move over.


@future
    public static void cloneAttachment(Id cattaList,List<String> cwwList)
    {
        Credit_Writeup__c cww = [select id,Credit_Writeup_Account__c from Credit_Writeup__c where id=:cattaList];
        List<Credit_Writeup__c> cwList = [select id from Credit_Writeup__c where id IN:cwwList];
        
        List<Contact> cList = new List<Contact>();       
    List<Lead> lList = new List<Lead>(); 
    List<Attachment> al = new List<Attachment>();
    List<Attachment> insertAttList = new List<Attachment>();
    Credit_Writeup__c Cw = new Credit_Writeup__c();                   
    //Id cont = String.valueOf(cww.Contact__c).substring(0,15);
  Id accnt =  String.valueOf(cww.Credit_Writeup_Account__c).substring(0, 15);
  system.debug('account is '+accnt);

    if(accnt != null){
    
    clist = [select  id , Contact_Id__c from contact where Merged_Contact__c = true AND  accountId= :accnt];
           }
           Id contid = cList[0].Contact_Id__c;

        
    if( cList.size() > 0  && cList != null) {
    
     
        lList = [SELECT convertedContactId FROM Lead WHERE  convertedContactId IN: cList];
    }
    
        if(cList[1].Merged_Contact__c = true)    
        {
           if(lList != null && lList.size()>0)
                  
           aL = [select id,body,name from Attachment where parentId =:contid  Limit 1] ;   //************* at this point iam confused unable to retreive the contact id which is the parent id//

                   
                  }  
       /* else{
            
                aL = [select id,body,name from Attachment where parentId= :lList[0].convertedContactId Limit 1] ;   
 
        }*/
    if(aL != null&&aL.size()>0){
   
     
        
        for(Credit_Writeup__c c: cwList)
        {
        
            
           
                   Attachment att = new Attachment(name = aL[0].name, body = aL[0].body, parentid = c.id);
                   insertAttList.add(att);
                
    
            
        }   
             
   
       if(insertAttList.size() > 0)
       {
            insert insertAttList;
       }
        
    }
        
        
        
        
        
        
        
    }

}