Describe IncludePublicFields as a fallback and write a shadowed field once

src/NLog/Internal/ObjectReflectionCache.cs

@@ -298,15 +298,32 @@ namespace NLog.Internal
         private static FastPropertyLookup[] BuildFieldLookup(FieldInfo[] fields)
         {
             FastPropertyLookup[] fastLookup = new FastPropertyLookup[fields.Length];
+            int count = 0;
             for (int i = 0; i < fields.Length; ++i)
             {
                 var field = fields[i];
+                if (IsHiddenByDerivedField(field, fields))
+                    continue;   // A derived type redeclared the name with "new", so the member is written once
+
                 TypeCode typeCode = Type.GetTypeCode(field.FieldType);
-                fastLookup[i] = new FastPropertyLookup(field.Name, typeCode, (o, p) => field.GetValue(o));
+                fastLookup[count++] = new FastPropertyLookup(field.Name, typeCode, (o, p) => field.GetValue(o));
             }
+
+            if (count != fastLookup.Length)
+                Array.Resize(ref fastLookup, count);
             return fastLookup;
         }
 
+        private static bool IsHiddenByDerivedField(FieldInfo field, FieldInfo[] fields)
+        {
+            foreach (var other in fields)
+            {
+                if (!ReferenceEquals(other, field) && other.Name == field.Name && other.DeclaringType != null && field.DeclaringType != null && other.DeclaringType.IsSubclassOf(field.DeclaringType))
+                    return true;
+            }
+            return false;
+        }
+
         private static FastPropertyLookup[] BuildFastLookup(PropertyInfo[] properties, bool includeType)
         {
             int fastAccessIndex = includeType ? 1 : 0;

src/NLog/Targets/JsonSerializeOptions.cs

@@ -106,7 +106,7 @@ namespace NLog.Targets
         public int MaxRecursionLimit { get; set; } = 10;
 
         /// <summary>
-        /// Should public fields be included in the JSON output, in addition to public properties.
+        /// Should public fields be included in the JSON output, for objects that have no public properties.
         /// </summary>
         /// <remarks>Default: <see langword="false"/></remarks>
         public bool IncludePublicFields { get; set; }

tests/NLog.UnitTests/Targets/DefaultJsonSerializerClassTests.cs

@@ -95,6 +95,16 @@ namespace NLog.UnitTests.Targets
             public string ReadPrivate() => _private;
         }
 
+        private class ClassWithBaseField
+        {
+            public string Name = "base";
+        }
+
+        private sealed class ClassWithShadowingField : ClassWithBaseField
+        {
+            public new string Name = "derived";
+        }
+
         private static ContainerClass BuildSampleObject()
         {
             var testObject = new ContainerClass
@@ -236,6 +246,12 @@ namespace NLog.UnitTests.Targets
             Assert.Equal(@"{""Name"":""John"",""Age"":42}", withFieldsAgain.ToString());
         }
 
+        [Fact]
+        public void IncludePublicFields_ShadowedField_IsWrittenOnce()
+        {
+            Assert.Equal(@"{""Name"":""derived""}", SerializeWithFields(new ClassWithShadowingField()));
+        }
+
         [Fact]
         public void IncludePublicFields_ExpandoObject_StillEnumeratesEntries()
         {