trigger Example123 on Case (before insert, before update) {   

  Set<Id> accountIdSet = new Set<Id>();

  for(Case caseForVarRecord : Trigger.new){

    if(caseForVarRecord.AccountId != null ){
      accountIdSet .add(caseForVarRecord.AccountId);
    }        
  }
  
  map<Id,Account> accountMap =  new Map<Id,Account>([Select Id, Ownerid from Account where Id =: accountIdSet ]);

  for (Case c: Trigger.new){ 
    if(c.AdditionalCon__c ==null ){
      c.AdditionalCon__c = accountMap.get(c.AccountId).OwnerId;               
    }
  } 
}

trigger Example123 on Case (before insert, before update) {

    public List <Account> lstAcc =  new List<Account>();
    public Id accountOwner;
   /* For (Case c : Trigger.new){
        lst.add(c.id);
    }*/
    //system.debug('******' +lst);
     list<case> cc= [select id,AccountId, case.Account.OwnerId, AdditionalCon__c from case where id =: Trigger.new];
     system.debug('******' +cc);
     
      for(Case cas : cc){
      lstAcc = [Select Id, Ownerid from Account where Id  =: cas.AccountId]; 
      }
     for (Case c: cc){ 
       // system.debug('c.account.Ownerid**********'+c.account.Ownerid);     
         if((Trigger.isInsert || Trigger.isUpdate ) && Trigger.isBefore ){  //Before Insert       
            //system.debug('c.account.Ownerid**********'+c.account.Ownerid);  
             if(c.AdditionalCon__c ==null ){

                for(Account a : lstAcc ){
                    if(c.AccountId == a.Id){
                    accountOwner = a.OwnerId;
                    }   
                } 
                 
               system.debug('AccountOwner**********'+accountOwner);  
               c.AdditionalCon__c = accountOwner; 
               system.debug('AdditionalCon :' +c.AdditionalCon__c);
           }
       } 
          //lst.add(c);
    }
    
        //update lst;
           }