title: A column named [column name] already belongs to this DataTable when form is reloaded
body: <p>I have a data grid view (designed for user input which is then made into a data table in the database). When I load the form, everything loads as I want it to. However, if I exit the form, and reload it I get the following error message:</p>
  
  <pre><code>A column named 'line' already belongs to this DataTable
  </code></pre>
  
  <p>'Line' being the first column name I have declared;</p>
  
  <pre><code>DataTable dt = new DataTable();
  #region loadtable
  public void newquoteForm_Load(object sender, EventArgs e)
  {
      getCompanyInfo();
      loadTable();
  }
  private void loadTable()
  {
      dt.Clear();
      DataRow dr;
      dt.Columns.Add("Line");
      dt.Columns.Add("Item Name");
      dt.Columns.Add("Item Description");
      dt.Columns.Add("Retail Price");
      dt.Columns.Add("Supplier Number");
      dt.Columns.Add("Quantity Required");
      dt.Columns.Add("In Stock");
      dt.Columns.Add("Cost Price");
      dt.Columns.Add("Total Cost");
      dt.Columns.Add("Total Retail");
      dt.Columns["Line"].AutoIncrement = true;
      dt.Columns["Line"].AutoIncrementSeed = -1;
      dt.Columns["Line"].AutoIncrementStep = 1;
      dt.Columns["Line"].ReadOnly = true;
      dt.Columns["Item Name"].ReadOnly = true;
      dt.Columns["Item Description"].ReadOnly = true;
      dt.Columns["Cost Price"].ReadOnly = true;
      dt.Columns["In Stock"].ReadOnly = true;
      dt.Columns["Retail Price"].ReadOnly = true;
      dt.Columns["Supplier Number"].ReadOnly = true;
  
      dr = dt.NewRow();
      dataGridView1.DataSource = dt;
  }
  </code></pre>
  
  <p>How can I prevent this from happening?</p>