highlight datagridview column if there is duplicated cell

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace DGV_Tester
{
	public partial class Form1 : Form
	{
		DataGridViewCellStyle m_hilightStyle = null;

		public Form1()
		{
			InitializeComponent();
			SetHilightCellStyle();
		}

		private void SetHilightCellStyle()
		{
			m_hilightStyle = dataGridView1.DefaultCellStyle.Clone();
			m_hilightStyle.ForeColor = Color.Red;
			m_hilightStyle.BackColor = Color.Yellow;
		}

		private void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e)
		{
			ValidateGrid();
		}

		private void Form1_Load(object sender, EventArgs e)
		{
			dataGridView1.DataSource = MyData.Values;
			ValidateGrid();
		}

		private void ValidateGrid()
		{
			foreach (var colNum in Enumerable.Range(0, dataGridView1.Columns.Count))
			{
				bool matchFound = false;
				string keyValue = null;

				// Row counting math: say we have two rows - RowCount = 3; Range() yields three values - 0, 1 and two
				// targetRowNum range must be '1 for two rows, 2 for one row and 3 for no rows', 
				// i.e. the Range() count must be RowCount - 1 - rowNum
				foreach (var rowNum in Enumerable.Range(0, dataGridView1.RowCount))
				{
					keyValue = dataGridView1.Rows[rowNum].Cells[colNum].Value.ToString();
					foreach (var targetRowNum in Enumerable.Range(rowNum + 1, dataGridView1.RowCount - 1 - rowNum))
					
					if (keyValue == dataGridView1.Rows[targetRowNum].Cells[colNum].Value.ToString())
					{
						matchFound = true;
						break;
					}
				}
				HighlightColumn(colNum, matchFound);
			}
		}

		private void HighlightColumn(int colNum, bool matchFound)
		{
			dataGridView1.Columns[colNum].DefaultCellStyle = matchFound ? m_hilightStyle : dataGridView1.DefaultCellStyle;
		}
	}
}