Interesting question. I played around with this for a while and discovered that while a JObject cannot contain properties with duplicate names, the JsonTextReader used to populate it during deserialization does not have such a restriction. (This makes sense if you think about it: it's a forward-only reader; it is not concerned with what it has read in the past). Armed with this knowledge, I took a shot at writing some code that will populate a hierarchy of JTokens, converting property values to JArrays as necessary if a duplicate property name is encountered in a particular JObject. Since I don't know your actual JSON and requirements, you may need to make some adjustments to it, but it's something to start with at least.

While a JObject cannot contain properties with duplicate names, the JsonTextReader used to populate it during deserialization does not have such a restriction (this makes sense if you think about it: it's a forward-only reader; it is not concerned with what it has read in the past).

Here is some code that will populate a hierarchy of JTokens, converting property values to JArrays as necessary if a duplicate property name is encountered in a particular JObject.

Since I don't know your actual JSON and requirements, you may need to make some adjustments to it, but it's something to start with at least.

public static JToken DeserializeAndCombineDuplicates(JsonTextReader reader) { if (reader.TokenType == JsonToken.None) { reader.Read(); } if (reader.TokenType == JsonToken.StartObject) { reader.Read(); JObject obj = new JObject(); while (reader.TokenType != JsonToken.EndObject) { string propName = (string)reader.Value; reader.Read(); JToken newValue = DeserializeAndCombineDuplicates(reader); JToken existingValue = obj[propName]; if (existingValue == null) { obj.Add(new JProperty(propName, newValue)); } else if (existingValue.Type == JTokenType.Array) { CombineWithArray((JArray)existingValue, newValue); } else // Convert existing non-array property value to an array { JProperty prop = (JProperty)existingValue.Parent; JArray array = new JArray(); prop.Value = array; array.Add(existingValue); CombineWithArray(array, newValue); } reader.Read(); } return obj; } if (reader.TokenType == JsonToken.StartArray) { reader.Read(); JArray array = new JArray(); while (reader.TokenType != JsonToken.EndArray) { array.Add(DeserializeAndCombineDuplicates(reader)); reader.Read(); } return array; } return new JValue(reader.Value); } private static void CombineWithArray(JArray array, JToken value) { if (value.Type == JTokenType.Array) { foreach (JToken child in value.Children()) array.Add(child); } else { array.Add(value); } } 

class Program { static void Main(string[] args) { string json = @" { ""Foo"" : 1, ""Foo"" : [2], ""Foo"" : [3, 4], ""Bar"" : { ""X"" : [ ""A"", ""B"" ] }, ""Bar"" : { ""X"" : ""C"", ""X"" : ""D"" }, }"; using (StringReader sr = new StringReader(json)) using (JsonTextReader reader = new JsonTextReader(sr)) { JToken token = DeserializeAndCombineDuplicates(reader); Dump(token, ""); } } private static void Dump(JToken token, string indent) { Console.Write(indent); if (token == null) { Console.WriteLine("null"); return; } Console.Write(token.Type); if (token is JProperty) Console.Write(" (name=" + ((JProperty)token).Name + ")"); else if (token is JValue) Console.Write(" (value=" + token.ToString() + ")"); Console.WriteLine(); if (token.HasValues) foreach (JToken child in token.Children()) Dump(child, indent + " "); } } 

public static JToken DeserializeAndCombineDuplicates(JsonTextReader reader) { if (reader.TokenType == JsonToken.None) { reader.Read(); } if (reader.TokenType == JsonToken.StartObject) { reader.Read(); JObject obj = new JObject(); while (reader.TokenType != JsonToken.EndObject) { string propName = (string)reader.Value; reader.Read(); JToken newValue = DeserializeAndCombineDuplicates(reader); JToken existingValue = obj[propName]; if (existingValue == null) { obj.Add(new JProperty(propName, newValue)); } else if (existingValue.Type == JTokenType.Array) { CombineWithArray((JArray)existingValue, newValue); } else // Convert existing non-array property value to an array { JProperty prop = (JProperty)existingValue.Parent; JArray array = new JArray(); prop.Value = array; array.Add(existingValue); CombineWithArray(array, newValue); } reader.Read(); } return obj; } if (reader.TokenType == JsonToken.StartArray) { reader.Read(); JArray array = new JArray(); while (reader.TokenType != JsonToken.EndArray) { array.Add(DeserializeAndCombineDuplicates(reader)); reader.Read(); } return array; } return new JValue(reader.Value); } private static void CombineWithArray(JArray array, JToken value) { if (value.Type == JTokenType.Array) { foreach (JToken child in value.Children()) array.Add(child); } else { array.Add(value); } } 

class Program { static void Main(string[] args) { string json = @" { ""Foo"" : 1, ""Foo"" : [2], ""Foo"" : [3, 4], ""Bar"" : { ""X"" : [ ""A"", ""B"" ] }, ""Bar"" : { ""X"" : ""C"", ""X"" : ""D"" }, }"; using (StringReader sr = new StringReader(json)) using (JsonTextReader reader = new JsonTextReader(sr)) { JToken token = DeserializeAndCombineDuplicates(reader); Dump(token, ""); } } private static void Dump(JToken token, string indent) { Console.Write(indent); if (token == null) { Console.WriteLine("null"); return; } Console.Write(token.Type); if (token is JProperty) Console.Write(" (name=" + ((JProperty)token).Name + ")"); else if (token is JValue) Console.Write(" (value=" + token.ToString() + ")"); Console.WriteLine(); if (token.HasValues) foreach (JToken child in token.Children()) Dump(child, indent + " "); } }