如何突出显示重复的列？

问：

这就是我到目前为止所拥有的，但我只是对它的工作原理感到困惑。

protected void GridView1_PreRender(object sender, EventArgs e)
{
  HashSet<string> uniqueValues = new HashSet<string>();
  foreach (GridViewRow row in GridView1.Rows)
  {
    // checking duplicate staring at column 1
    string cellText = row.Cells[1].Text;
    // Check if we've already seen this value
    if (uniqueValues.Contains(cellText)) 
    {
      row.Cells[0].CssClass = "duplicate-highlight";
      // This value is a duplicate, apply a CSS class
      // need to add duplicate file to css file standardDesign.cs
    }
    else
    {
      // It's not a duplicate, add it to the set of seen values
      uniqueValues.Add(cellText); 
    }
  }
}

我试图让它突出显示重复的列。