How Do I Validate Textboxes Values To Be Unique using C#

private const string Exclam = "!";

private TextBox leftTextBox;

private bool tbOneHasExclam = false;

private string leftTextBoxOldContent;

private string leftTextBoxNewContent;

private Dictionary<TextBox, string> dctTBToStr = new Dictionary<TextBox, string>();

private void YourMainForm_Load(object sender, EventArgs e)
{
    foreach (TextBox tb in TBPanel.Controls.OfType<textbox>())
    {
        // connect a 'Leave EventHandler
        tb.Leave += TextBoxes_Leave;

        // add to Dictionary
        dctTBToStr.Add(tb, String.Empty);
    }
}

private void TextBoxes_Leave(object sender, EventArgs e)
{
    leftTextBox = sender as TextBox;

    leftTextBoxOldContent = dctTBToStr[leftTextBox];
    leftTextBoxNewContent = leftTextBox.Text;
    
    // if the content hasn't changed, keep going
    if (leftTextBoxOldContent == leftTextBoxNewContent) return;

    // if the new content is "" then update and keep going
    if(leftTextBoxNewContent == String.Empty)
    {
        dctTBToStr[leftTextBox] = String.Empty;
        return;
    }

    // now the fun begins !
    CheckForDuplicateEntry();
}

private void CheckForDuplicateEntry()
{
    switch (leftTextBox.Name)
    {
        // TextBox1 has changed
        case "TextBox1":
            // update
            dctTBToStr[TextBox1] = leftTextBoxNewContent;

            // test for "!"
            tbOneHasExclam = leftTextBoxNewContent == Exclam;

            // if TextBoxOne is "" or "!"
            // we can keep going
            if (tbOneHasExclam || leftTextBoxNewContent == String.Empty) break;

            // what needs to happen here ?
            // we know that TextBox1 is not "!"
            // we need to find any other TextBoxes with
            // text of "!" and the one possible TextBox
            // where its text == TextBox1.Text

            // build a collection of TextBoxes to be cleared
            // in order to avoid getting a modify collection error:
            // we can't modify a Dictionary value in a 'for or 'foreach loop

            List<textbox> duplicates = null;

            foreach (TextBox tb in dctTBToStr.Keys)
            {
                // ignore what can be skipped over
                if (tb.Text == String.Empty || tb == TextBox1) continue;

                if (tb.Text == leftTextBoxNewContent || tb.Text == Exclam)
                {
                    duplicates.Add(tb);
                }
            }

            if (duplicates != null)
            {
                for (int i = 0; i < duplicates.Count; i++)
                {
                    TextBox duplicate = duplicates[i];
                    duplicate.Clear();
                    dctTBToStr[duplicate] = String.Empty; 
                }
            }
           break;

        // process change in all other TextBoxes
        default:

           // and now ... you write the code
           break;
    }
}

public partial class Form2 : Form
    {
        int totalCtrl = 3;
        public Form2()
        {
            InitializeComponent();
        }

        private void HCFA_21_Diag1_Leave(object sender, EventArgs e)
        {
            if (IsDuplicate("1")) 
            MessageBox.Show("Is duplicate value!");
        }

        private bool IsDuplicate(string keyedID)
        {

            bool isDuplicate = false;
            string fieldName =  "HCFA_21_Diag";
            Control ctrl = this.Controls[fieldName + keyedID];

            if (ctrl.Text != "!" && ctrl.Text != string.Empty)
            {
                for (int i = 1; i <= totalCtrl; i++)
                {
                    Control allCtrl = this.Controls[fieldName + i.ToString()];

                    if (ctrl.Text == allCtrl.Text && ctrl.Name != allCtrl.Name)
                    {
                        isDuplicate = true;
                        break;
                    }
                }
            }


            return isDuplicate;

        
        }

        private void HCFA_21_Diag2_Leave(object sender, EventArgs e)
        {
            if (IsDuplicate("2")) 
            MessageBox.Show("Is duplicate value!");
        }

        private void HCFA_21_Diag3_Leave(object sender, EventArgs e)
        {
             if (IsDuplicate("3")) 
            MessageBox.Show("Is duplicate value!");
        }


    }
}

RequiredFieldValidator1.ErrorMessage = "";
        Label9.Text = "";
        string name = TextBox7.Text.ToString();
       // string constr = "data Source=MURALY-PC\\SQLEXPRESS; database=Online; Integrated Security=SSPI";
       // SqlConnection con = new SqlConnection(constring);
        con.Open();
        string query = "select username from EMPP1 where username='" + TextBox7.Text+ "'";
        SqlCommand cmd = new SqlCommand(query, con);
        SqlDataReader dr = cmd.ExecuteReader();
        dr.Read();
        if (dr.HasRows)
        {
            Label11.Text = "user name is already exist";
            TextBox7.Text = "";
            dr.Close();
          
        }
        else
        {
            dr.Close();
            Label11.Text = "";
            Label11.Text = "username is available";
                     
        }
        con.Close();