combo box to grid view(select only one time)

for (int i = 0; i < dataGridView1.Rows.Count; i++)
{
    if (combobox.Text == dataGridView1.Rows[i].Cells[0].Value.ToString())
    {
        MessageBox.Show("duplicate value");
        return;
    }
}

dataGridView1.Rows.Add(textBox1.Text.Trim());

You can add Your code to add button