[java] Fix duplicate field name handling in InstanceCoercer#getFieldWriters

java/src/org/openqa/selenium/json/InstanceCoercer.java

@@ -126,6 +126,10 @@ private Map<String, TypeAndWriter> getFieldWriters(Constructor<?> constructor) {
                         }
                       };
                   return new TypeAndWriter(type, writer);
+                },
+                (existing, replacement) -> {
+                  throw new JsonException(
+                      "Duplicate JSON field name detected while collecting field writers");
                 }));
   }
 

java/test/org/openqa/selenium/json/JsonTest.java

@@ -132,6 +132,18 @@ void shouldAllowUserToPopulateFieldsDirectly() {
     assertThat(seen.theName).isEqualTo("fishy");
   }
 
+  @Test
+  void shouldThrowWhenDuplicateFieldNamesExistWithFieldSetting() {
+    String raw = "{\"value\": \"test\"}";
+
+    assertThatThrownBy(() -> new Json().toType(raw, ChildFieldBean.class, BY_FIELD))
+        .isInstanceOf(JsonException.class)
+        .hasMessageStartingWith("Unable to parse: " + raw)
+        .cause()
+        .isInstanceOf(JsonException.class)
+        .hasMessage("Duplicate JSON field name detected while collecting field writers");
+  }
+
   @Test
   void settingFinalFieldsShouldWork() {
     Map<String, String> map = Map.of("theName", "fishy");
@@ -661,6 +673,14 @@ public void setBean(SimpleBean bean) {
     }
   }
 
+  public static class ParentFieldBean {
+    String value;
+  }
+
+  public static class ChildFieldBean extends ParentFieldBean {
+    String value;
+  }
+
   public static class JsonAware {
     private final String convertedValue;