restore behavior test for duplicate JSON keys with clearer naming and comment

framework/test/Volo.Abp.Localization.Tests/Volo/Abp/Localization/JsonLocalizationDictionaryBuilder_Tests.cs

@@ -9,6 +9,29 @@ namespace Volo.Abp.Localization;
 /// </summary>
 public class JsonLocalizationDictionaryBuilder_Tests
 {
+    [Fact]
+    public void Should_Use_Last_Value_When_Json_Contains_Duplicate_Keys()
+    {
+        // This test locks the behavior of System.Text.Json when deserializing duplicate JSON property names.
+        // If STJ changes this behavior in a future version, this test will catch the regression.
+        var input = """
+            {
+              "culture": "en",
+              "texts": {
+                "ThisFieldIsRequired": "This field is required",
+                "MaxLengthErrorMessage": "This field can be maximum of '{0}' chars",
+                "ThisFieldIsRequired": "This field is required again"
+              }
+            }
+            """;
+
+        var localizationDictionary = JsonLocalizationDictionaryBuilder.BuildFromJsonString(input);
+        localizationDictionary.ShouldNotBeNull();
+        var localizationString = localizationDictionary.GetOrNull("ThisFieldIsRequired");
+        localizationString.ShouldNotBeNull();
+        localizationString.Value.ShouldBe("This field is required again");
+    }
+
     [Fact]
     public void Should_Use_Nested_Value_When_Flat_Key_Is_Defined_Before_Nested_Object()
     {