Responses responses = Responses.build().filter(ResponseFilter.ALL);
Map<String,Response> map = new HashMap<>();
int c = 0;
for (Response response:responses) {
String id = response.getRespondentID();
Response response2 = map.put(id,response);
if (response2!=null) {
System.out.println("" + (++c) + " Detected two responses with same id " + id);
}
}
}