diff --git a/software/cityjson-valschema/c++11/CMakeLists.txt b/software/cityjson-valschema/c++11/CMakeLists.txt
index f92baf3..ef8368c 100644
--- a/software/cityjson-valschema/c++11/CMakeLists.txt
+++ b/software/cityjson-valschema/c++11/CMakeLists.txt
@@ -2,17 +2,19 @@ cmake_minimum_required (VERSION 3.2)
 project (cityjson-valschema)
 
 set(CMAKE_CXX_COMPILER g++)
-set(CMAKE_CXX_FLAGS "-Wall -std=c++11")
+set(CMAKE_CXX_FLAGS "-Wall -std=c++11 -Wno-unknown-pragmas")
 
 set(${PROJECT_NAME}_VERSION_MAJOR 1)
 set(${PROJECT_NAME}_VERSION_MINOR 0)
 
+
 include_directories(include)
 set(SOURCES src/main.cpp)
 
 add_library(json-schema-validator SHARED
     include/json-schema-draft4.json.cpp
     include/json-uri.cpp
+    include/jsoncpp.cpp
     include/json-validator.cpp)
 
 add_executable(${PROJECT_NAME} ${SOURCES} src/main.cpp)
diff --git a/software/cityjson-valschema/c++11/src/main.cpp b/software/cityjson-valschema/c++11/src/main.cpp
index abcc02a..6db875e 100644
--- a/software/cityjson-valschema/c++11/src/main.cpp
+++ b/software/cityjson-valschema/c++11/src/main.cpp
@@ -20,6 +20,7 @@ __   ____ _| |___  ___| |__   ___ _ __ ___   __ _
 */
 
 #include <json-schema.hpp>
+#include "json/json.h"
 #include <fstream>
 #include <iostream>
 #include <sstream>
@@ -32,6 +33,7 @@ using nlohmann::json_schema_draft4::json_validator;
 
 static void loader(const json_uri &uri, json &schema);
 bool is_valid_json_schema(json& j, json& jschema, std::stringstream& ss);
+bool is_valid_duplicate_keys(const char* nameinput);
 bool is_valid_citygml_attributes(json& j, json& jschema);
 bool is_valid_metadata(json& j, json& jschema);
 bool is_valid_semantics(json& j);
@@ -44,7 +46,7 @@ int main(int argc, char *argv[]) {
   bool isValid = true;
   bool woWarnings = true;
   json j;
-  const char* nameinput = (argc > 1) ? argv[1] : "/Users/hugo/temp/example2.json";
+  const char* nameinput = (argc > 1) ? argv[1] : "../../../../example-datasets/dummy-values/example2.json";
   std::ifstream input2(nameinput);
   input2 >> j;
   std::cout << "Input file: " << nameinput << std::endl;
@@ -74,10 +76,16 @@ int main(int argc, char *argv[]) {
   }
 
   std::stringstream ss;
+  //-- validate the schema
   if ( (isValid == true) && (is_valid_json_schema(j, jschema, ss) == false) ){
     std::cout << ss.str() << std::endl;
     isValid = false;
   }
+  //-- check for duplicate keys, which is impossible with schema and/or my parsing library
+  if ( (isValid == true) && (is_valid_duplicate_keys(nameinput) == false) ){
+    std::cout << "ERROR:   Duplicate IDs (keys) for the City Objects." << std::endl;  
+    isValid = false;
+  }
   if (isValid == true) {
     if (is_valid_building_parts(j) == false)
       isValid = false;
@@ -93,16 +101,6 @@ int main(int argc, char *argv[]) {
       woWarnings = false;
   }
 
-  //-- unique keys for CO?
-  std::cout << "=====" << std::endl;
-  for (json::iterator coit = j["CityObjects"].begin(); coit != j["CityObjects"].end(); ++coit) {
-    std::cout << coit.key() << std::endl;
-    if (coit.key() == "2929")
-      std::cout << coit.value() << std::endl;
-  }
-  std::cout << "=====" << std::endl;
-
-
   std::cout << std::endl;
   if (isValid == true) {
     std::cout << "File is VALID";
@@ -119,6 +117,18 @@ int main(int argc, char *argv[]) {
 }
 
 
+bool is_valid_duplicate_keys(const char* nameinput) {
+  Json::CharReaderBuilder builder;
+  builder["rejectDupKeys"] = true;
+  JSONCPP_STRING errs;
+  Json::Value j;   // 'root' will contain the root value after parsing.
+  // const char* nameinput = "/Users/hugo/projects/cityjson-new/example-datasets/dummy-values/example3.json";
+  std::ifstream input(nameinput);
+  bool ok = parseFromStream(builder, input, &j, &errs);
+  return ok;
+}
+
+
 bool is_valid_building_pi_parent(json& j) {
   bool isValid = true;
   std::set<std::string> pis;