JamesNK/Newtonsoft.Json · error · ArgumentException

Can not add property {0} to {1}. Property with the same name

Error message

Can not add property {0} to {1}. Property with the same name already exists on object.

What it means

ArgumentException thrown by JObject.ValidateToken when adding a JProperty whose name already exists in the JObject. By default JObject does not allow duplicate property names; during normal Add/Insert (not merge) a name collision is rejected.

Source

Thrown at Src/Newtonsoft.Json/Linq/JObject.cs:177

            {
                throw new ArgumentException("Can not add {0} to {1}.".FormatWith(CultureInfo.InvariantCulture, o.GetType(), GetType()));
            }

            JProperty newProperty = (JProperty)o;

            if (existing != null)
            {
                JProperty existingProperty = (JProperty)existing;

                if (newProperty.Name == existingProperty.Name)
                {
                    return;
                }
            }

            if (_properties.TryGetValue(newProperty.Name, out existing))
            {
                throw new ArgumentException("Can not add property {0} to {1}. Property with the same name already exists on object.".FormatWith(CultureInfo.InvariantCulture, newProperty.Name, GetType()));
            }
        }

        internal override void MergeItem(object content, JsonMergeSettings? settings)
        {
            if (!(content is JObject o))
            {
                return;
            }

            foreach (KeyValuePair<string, JToken?> contentItem in o)
            {
                JProperty? existingProperty = Property(contentItem.Key, settings?.PropertyNameComparison ?? StringComparison.Ordinal);

                if (existingProperty == null)
                {
                    Add(contentItem.Key, contentItem.Value);
                }

View on GitHub (pinned to 4f73e74372)

Solutions

1. Check existence first: if (jObject["x"] == null) jObject.Add("x", value); or use Property(name) == null.
2. Use the indexer jObject["x"] = value to replace an existing property instead of Add.
3. Use Merge with JsonMergeSettings if you need to combine two JObjects (Merge replaces/merges duplicates rather than throwing).

Example fix

// before
jObject.Add(new JProperty("id", 1));
jObject.Add(new JProperty("id", 2)); // ArgumentException

// after
jObject["id"] = 2; // replaces
// or
if (jObject.Property("id") == null) jObject.Add("id", 2);

static void Put(JObject obj, string name, JToken value)
{
    if (obj.Property(name) != null) obj[name] = value;
    else obj.Add(name, value);
}

static bool HasProperty(JObject obj, string name) => obj.Property(name) != null;

try { jObject.Add(new JProperty(name, value)); }
catch (ArgumentException ex) when (ex.Message.Contains("already exists"))
{
    jObject[name] = value;
}

When it happens

Trigger: Calling jObject.Add(new JProperty("x", 1)) when 'x' is already present; building a JObject programmatically and inserting two properties with the same name; copying properties from another object without checking for existing keys.

Common situations: Aggregating properties from multiple sources into one JObject; loops that add keys derived from data containing duplicates; converting dictionaries that may share keys.

Related errors

• Can not add {0} to {1}.
• Accessed JObject values with invalid key value: {0}. Object
• Set JObject values with invalid key value: {0}. Object prope
• Object serialized to {0}. JObject instance expected.
• array

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/fd47c9257db7c254. Report an issue: GitHub.