How to prevent a user from resubmitting a form?

I am developping a Single Page Application.

At the end of the application, the user gets to submit his contact information (name, phone number, etc). This sends an Email and modifies the page to a “Thanks for submitting […]” page.

The problem is, the client can press the Back button and REsend the Email.

Is there a way to prevent this sort of.. spam?

Code

Sub BT_Send(sender As Object, e As EventArgs) Handles BT_Send.Click
Try
    'Creating the Email Message
    Dim mailMessage As New MailMessage()
    mailMessage.To.Add("[email protected]")
    mailMessage.From = New MailAddress("[email protected]", "Robot")
    mailMessage.Subject = "Test"
    mailMessage.IsBodyHtml = True
    mailMessage.Body = LBL_Emailbody.Text & _
        "<br><br><br><div style=""font-size: 0.7em;"">Robot speaking, I will not answer if you send me a message.</div>"
    Dim smtpClient As New SmtpClient("Something.com")
    smtpClient.Send(mailMessage)


    PNL_Before.Visible = False
    PNL_After.Visible = True
Catch ex As Exception
    LBL_errorEmail.Visible = True
    'Should never happen...
End Try
End sub

Answers:

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Method 1

Here is a very simple example, where I use a static variable on page and avoid database.

the asp.net page is

<asp:Literal runat="server" ID="txtInfos"></asp:Literal><br />
<asp:TextBox runat="server" ID="txtEmail"></asp:TextBox><br />
<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />/>

and the code behind.

static Dictionary<string, DateTime> cLastSubmits = new Dictionary<string, DateTime>();

private static readonly object syncLock = new object();

protected void Button1_Click(object sender, EventArgs e)
{
    DateTime cWhenLast;

    lock (syncLock)
    {
        var cNowIs = DateTime.UtcNow;
        if (cLastSubmits.TryGetValue(txtEmail.Text, out cWhenLast))
        {
            if (cWhenLast > cNowIs )
            {
                txtInfos.Text = "Please contact us again after 10 seconds";
                return;
            }
            else
            {
                // ok I let him submit the form, but note the last date time.
                cLastSubmits.Remove(txtEmail.Text);
            }
        }

        foreach(var DelMe in cLastSubmits.Where(x => cNowIs > x.Value).ToList())
            cLastSubmits.Remove(DelMe.Key);

        // if reach here, note the last datetime of submit            
        cLastSubmits.Add(txtEmail.Text, cNowIs.AddSeconds(10));
    }

    // and submit the form.
    txtInfos.Text = "thank you for submit the form";
}

Some notes.

• If you have many pools (web garden), then this may left user to
  submit at the same time up to the pool you have.
• Of course if a user submit fake data this can not protect you, and thats why we can use:
  1. The honey pot trick.
  2. captcha
  3. Other control thats understands the bots
  4. Code that understand the re-submit.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0