java.util.ConcurrentModificationException Issue

While running the following code snippet at some scenario, I got the java.util.ConcurrentModificationException exception.
Please advice: How to avoid it? what can be the worse scenario which ConcurrentModificationException can create? will it occupy the CPU Usage %? Any pointer would be great!!!

Code java:

public Map someMethod()
{
Map actionDosMap =new HashMap();
List associatedActionsDos =new ArrayList();
List associatedActionsPerformanceDos =new ArrayList();
 
Iterator iterActions =associatedActions.iterator();
while (iterActions.hasNext())
{
Iterator reposIt = reposSet.iterator();
                        while (reposIt.hasNext()) {
                              RepoRow row = (RepoRow) reposIt.next();
                              Properties props = row.getProperties();
 
                              DoData performanceReportsDo = new DoData();
 
                              if ("PerformanceReports".equals(props.getStringValue(do_type))) {
                                    performanceReportsDo.setDoType(doType);
                                    performanceReportsDo.setDoId(doID);                                  
                                    associatedActionsPerformanceDos.add(performanceReportsDo);
                                          }                     
                                    else
                                    {
                              DoData doc = new DoData();
                              doc.setDoType(doType);
                              doc.setDoId(doID);                                                    
                              associatedActionsDos.add(doc);
                                    }
 
          }
 
}
actionDosMap.put("performanceReportsDo",associatedActionsPerformanceDos);
actionDosMap.put("actionsNonPerfoDo",associatedActionsDos);
return actionDosMap;
}