trigger CloneOpp on Opportunity( after insert, after update ) {

  Set<String> fields = Opportunity.getSobjectType().getDescribe().fields.getMap().keySet();
  Map<Id,Opportunity_clone__c> opp = new Map<Id,Opportunity_clone__c>();

  for(Opportunity record: Trigger.new) {
  Opportunity_clone__c oppclone = new Opportunity_clone__c();
  oppclone.id = record.Opportunity_clone__c;
    opp.put(record.Opportunity_clone__c,oppclone);
    for(String field:fields) {
      if(field.indexOf('__c')>-1) { // This is a custom field.
        try {
          opp.get(record.Opportunity_clone__c).put(field,record.get(field));
        } catch(exception e) { /* NOTE: This just means copy failed. */ }
      }
    }
  }
  update opp.values();
}