Implemented custom duplicate key handling

- Supports: throw an exception (by default), ignore, overwrite & merge into a JSONArray
 - With tests, 4/4 passed.

src/test/java/org/json/junit/JSONObjectDuplicateKeyTest.java

@@ -0,0 +1,47 @@
+package org.json.junit;
+
+import org.json.*;
+
+import static org.junit.Assert.*;
+
+import org.junit.Test;
+
+public class JSONObjectDuplicateKeyTest {
+    private static final String TEST_SOURCE = "{\"key\": \"value1\", \"key\": \"value2\", \"key\": \"value3\"}";
+
+    @Test(expected = JSONException.class)
+    public void testThrowException() {
+        new JSONObject(TEST_SOURCE);
+    }
+
+    @Test
+    public void testIgnore() {
+        JSONObject jsonObject = new JSONObject(TEST_SOURCE, new JSONParserConfiguration(
+                JSONDuplicateKeyStrategy.IGNORE
+        ));
+
+        assertEquals("duplicate key shouldn't be overwritten", "value1", jsonObject.getString("key"));
+    }
+
+    @Test
+    public void testOverwrite() {
+        JSONObject jsonObject = new JSONObject(TEST_SOURCE, new JSONParserConfiguration(
+                JSONDuplicateKeyStrategy.OVERWRITE
+        ));
+
+        assertEquals("duplicate key should be overwritten", "value3", jsonObject.getString("key"));
+    }
+
+    @Test
+    public void testMergeIntoArray() {
+        JSONObject jsonObject = new JSONObject(TEST_SOURCE, new JSONParserConfiguration(
+                JSONDuplicateKeyStrategy.MERGE_INTO_ARRAY
+        ));
+
+        JSONArray jsonArray;
+        assertTrue("duplicate key should be merged into JSONArray", jsonObject.get("key") instanceof JSONArray
+                && (jsonArray = jsonObject.getJSONArray("key")).length() == 3
+                && jsonArray.getString(0).equals("value1") && jsonArray.getString(1).equals("value2")
+                && jsonArray.getString(2).equals("value3"));
+    }
+}