Specify precedence when a subtype has multiple labels

adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java

@@ -100,7 +100,8 @@ import javax.annotation.Nullable;
  *
  * <p>If an unknown subtype is encountered when decoding, this will throw a {@link
  * JsonDataException}. If an unknown type is encountered when encoding, this will throw an {@link
- * IllegalArgumentException}.
+ * IllegalArgumentException}. If the same subtype has multiple labels the first one is used when
+ * encoding.
  *
  * <p>If you want to specify a custom unknown fallback for decoding, you can do so via
  * {@link #withDefaultValue(Object)}. This instance should be immutable, as it is shared.

adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java

@@ -176,6 +176,8 @@ public final class PolymorphicJsonAdapterFactoryTest {
         .isEqualTo(new Success("Data!"));
     assertThat(adapter.fromJson("{\"type\":\"error\",\"error_logs\":{\"order\":66}}"))
         .isEqualTo(new Error(Collections.<String, Object>singletonMap("order", 66d)));
+    assertThat(adapter.toJson(new Success("Data!")))
+        .isEqualTo("{\"type\":\"success\",\"value\":\"Data!\"}");
   }
 
   @Test public void uniqueLabels() {