Changed Gson behavior to reject duplicate fields in a class.

gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapter.java

@@ -174,7 +174,11 @@ public final class ReflectiveTypeAdapter<T> extends TypeAdapter<T>  {
             Type fieldType = $Gson$Types.resolve(type.getType(), raw, field.getGenericType());
             BoundField boundField = createBoundField(context, field, getFieldName(raw, field, declaredType),
                 TypeToken.get(fieldType), serialize, deserialize);
-            result.put(boundField.name, boundField);
+            BoundField previous = result.put(boundField.name, boundField);
+            if (previous != null) {
+              throw new IllegalArgumentException(declaredType
+                  + " declares multiple JSON fields named " + previous.name);
+            }
           }
         }
         type = TypeToken.get($Gson$Types.resolve(type.getType(), raw, raw.getGenericSuperclass()));

gson/src/test/java/com/google/gson/functional/NamingPolicyTest.java

@@ -103,13 +103,12 @@ public class NamingPolicyTest extends TestCase {
 
   public void testGsonDuplicateNameUsingSerializedNameFieldNamingPolicySerialization() {
     Gson gson = builder.create();
-    ClassWithDuplicateFields target = new ClassWithDuplicateFields(10);
-    String actual = gson.toJson(target);
-    assertEquals("{\"a\":10}", actual);
-
-    target = new ClassWithDuplicateFields(3.0D);
-    actual = gson.toJson(target);
-    assertEquals("{\"a\":3.0}", actual);
+    try {
+      ClassWithDuplicateFields target = new ClassWithDuplicateFields(10);
+      gson.toJson(target);
+      fail();
+    } catch (IllegalArgumentException expected) {
+    }
   }
 
   public void testGsonWithUpperCamelCaseSpacesPolicySerialiation() {