Trigger
======
trigger CaseTrigger on Case (Before Insert){

   CaseTriggerUtils.processInsert(Trigger.new);

}


Class
=====
public class CaseTriggerUtils {
    
    public static void processInsert(list<Case> cseLst) {        
        processCase(cseLst, new Set<String>{'FW:', 'RE:', 'RW:'});  //Keywords are passed     
    }   
   
 public static void processCase(list<Case> cseLst, Set<String> keyWords) {
        String CaseSubject;
        String CaseId;
        String CaseOwnr;
       
        Map<String, List<String>> keyWordSubjectListMap = new Map<String,List<String>>();
        list<Case> caseLst = new list<Case>();
       
        for (Case c : cseLst) {    
            CaseId = c.id;   
            if(c.subject <> null){
                for(String keyWord : keyWords) {
                    if(c.Subject.contains(keyWord)) {
                        CaseSubject = c.subject.replaceAll(keyWord,'');
                        List<String> subjectList = keyWordSubjectListMap.containsKey(keyWord) ?
                        keyWordSubjectListMap.get(keyWord) : new List<String>();
                        subjectList.add(CaseSubject);
                        keyWordSubjectListMap.put(keyWord, subjectList);
                        break;
                    }
                     if(CaseSubject == null)  
                       CaseSubject = c.subject;
                }
            }
        }
        System.debug('Case Subject' +  CaseSubject);
        System.debug('MAP' + keyWordSubjectListMap);
        
       list<case> cseQry = [select id, casenumber,subject,createddate,ownerid,owner.name from case where CreatedDate = this_year and id <> :CaseId and subject like :('%' + CaseSubject + '%') order by createddate asc limit 10];
        //system.debug('Existing Cases :'  + cs.casenumber + ' ' + cs.subject + ' ' + cs.createddate + ' ' + cs.owner.name); 
        
       for(Case cs: cseLst){     
         if(!cseQry.isEmpty()){  
           if(cs.ownerid <> cseQry[0].ownerid)   // if owner is different from the current owner, then update the owner and set duplicate flag.
             cs.ownerid = cseQry[0].ownerid;   
             cs.Duplicate_With__c = cseQry[0].casenumber;   
           if ( cs.subject.length() > 18 ){    //keyword check is more than 18 character remove keyword and save.
              cs.subject = CaseSubject;
            } 
          }     
       }
        
 }
    
}