How do I pervent data posting twice from my form to the database.

[maluda]

Here is my code.(submit botton) whenever I refresh,the previous data posts back to the database.how do I prevent this please.Also,how do I prevent posting existing to database.

 Partial Class Contact
    Inherits System.Web.UI.PageProtected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim maluda As New SqlDataSource
        With maluda
            .ConnectionString = ConfigurationManager.ConnectionStrings("unique2").ConnectionString
            .InsertCommandType = SqlDataSourceCommandType.Text
            .InsertCommand = "insert into dbcontact(Fname,Email,Phone,Gender,State,Subject,Msg,Cdate) values (@Fname,@Email,@Phone,@Gender,@State,@Subject,@Msg,@Cdate)"
            .InsertParameters.Add("Fname", txtfname.Text)
            .InsertParameters.Add("Email", txtemail.Text)
            .InsertParameters.Add("Phone", txtPN.Text)
            .InsertParameters.Add("Gender", txtgender.SelectedItem.ToString)
            .InsertParameters.Add("State", txtstate.SelectedItem.ToString)
            .InsertParameters.Add("Subject", txtsubject.Text)
            .InsertParameters.Add("Msg", txtmsg.Text)
            .InsertParameters.Add("Cdate", Date.Today)
 
        End With
        Dim luda As String
        luda = maluda.Insert()
 
        'lblsucess.Text = "Delivered"
        UserMsgBox(sMsg:="Msg sent...click ok to continue")
 
        txtfname.Text = ""
        txtemail.Text = ""
        txtPN.Text = ""
        txtsubject.Text = ""
        txtmsg.Text = ""
 End Sub
 
 
 
 
 
 
    Public Sub UserMsgBox(ByVal sMsg As String)
 
        Dim sb As New StringBuilder()
        Dim oFormObject As System.Web.UI.Control
 
        sMsg = sMsg.Replace("'", "\'")
        sMsg = sMsg.Replace(Chr(34), "\" & Chr(34))
        sMsg = sMsg.Replace(vbCrLf, "\n")
        sMsg = "<script language=javascript>alert(""" & sMsg & """)</script>"
 
        sb = New StringBuilder()
        sb.Append(sMsg)
 
        For Each oFormObject In Me.Controls
            If TypeOf oFormObject Is HtmlForm Then
                Exit For
            End If
        Next
 
 
        oFormObject.Controls.AddAt(oFormObject.Controls.Count, New LiteralControl(sb.ToString()))
 
    End Sub