dotnet/machinelearning · error · ArgumentException

string.Format(Strings.DuplicateColumnName, column.Name)

Error message

string.Format(Strings.DuplicateColumnName, column.Name)

What it means

Thrown when a column being inserted into a DataFrameColumnCollection has a name that already exists in the DataFrame. The library enforces unique column names because columns are looked up by name via an internal name-to-index dictionary. It is an ArgumentException naming the offending 'column' parameter.

Source

Thrown at src/Microsoft.Data.Analysis/DataFrameColumnCollection.cs:87

        protected override void InsertItem(int columnIndex, DataFrameColumn column)
        {
            column = column ?? throw new ArgumentNullException(nameof(column));

            if (Count == 0)
            {
                //change RowCount on inserting first row to dataframe
                RowCount = column.Length;
            }
            else if (column.Length != RowCount)
            {
                //check all columns in the dataframe have the same length (amount of rows)
                throw new ArgumentException(Strings.MismatchedColumnLengths, nameof(column));
            }

            if (_columnNameToIndexDictionary.ContainsKey(column.Name))
            {
                throw new ArgumentException(string.Format(Strings.DuplicateColumnName, column.Name), nameof(column));
            }

            column.AddOwner(this);

            RowCount = column.Length;

            _columnNameToIndexDictionary[column.Name] = columnIndex;
            for (int i = columnIndex; i < Count; i++)
            {
                _columnNameToIndexDictionary[this[i].Name]++;
            }
            base.InsertItem(columnIndex, column);
            ColumnsChanged?.Invoke();
        }

        protected override void SetItem(int columnIndex, DataFrameColumn column)
        {
            column = column ?? throw new ArgumentNullException(nameof(column));

View on GitHub (pinned to 7b76e69cf9)

Solutions

1. Rename the new column to a unique name before inserting (set column.Name or create the column with a distinct name).
2. Check existence first with collection.Contains(name) or TryGetColumnIndex and skip/rename/remove the existing column.
3. Use DataFrame.AddColumn only when the name is guaranteed unique; dedupe source headers on load (e.g. suffix duplicates).

Example fix

// before
df.Columns.Add(new StringDataFrameColumn("Name", df.Rows.Count)); // 'Name' already exists
// after
if (!df.Columns.Contains("Name"))
    df.Columns.Add(new StringDataFrameColumn("Name", df.Rows.Count));
else
    df.Columns.Add(new StringDataFrameColumn("Name_2", df.Rows.Count));

if (df.Columns.Contains(newColumn.Name))
    throw new InvalidOperationException($"Column '{newColumn.Name}' already exists");
df.Columns.Add(newColumn);

bool CanAdd(DataFrameColumn c, DataFrame df) => c != null && !df.Columns.Contains(c.Name);

try
{
    df.Columns.Add(column);
}
catch (ArgumentException ex) when (ex.Message.Contains("already exists"))
{
    // rename or skip
}

When it happens

Trigger: Calling Insert, Add, or any API that internally inserts a second column whose DataFrameColumn.Name matches an existing column (the check is _columnNameToIndexDictionary.ContainsKey(column.Name)).

Common situations: Renaming a column to a name that already exists before inserting, loading two files/tables with duplicate headers and appending both, cloning a column and adding it back without renaming, joining datasets with overlapping column names.

Related errors

• Strings.DuplicateColumnName (formatted with column.Name)
• Value name '{0}' matches an existing column name
• {0} and {1} must be different
• Strings.InvalidColumnName (formatted with columnName)
• Strings.BadColumnCast (formatted with column.DataType, typeo

AI-assisted analysis of dotnet/machinelearning@7b76e69cf9 (2026-09-11). Data as JSON: /api/errors/8eb6913e3dd7233e. Report an issue: GitHub.