If a user tries to hit an ASP.NET submit button more than once only the very first request should submit to server.
All other clicks shouldn't make any request. So, for this you need to disable the button when first time click on button and enable it after processed.

Example 1)  in codebehind:
   
   /// <summary>
   /// OnInit stage
   /// </summary>
   /// <param name="e"></param>
   protected override void OnInit(EventArgs e)
   {
        base.OnInit(e);
        btnDataSave.Attributes.Add("onclick", "javascript:" + btnDataSave.ClientID + ".disabled=true;"  + ClientScript.GetPostBackEventReference(btnDataSave, string.Empty));
    }
  
  The method "ClientScript.GetPostBackEventReference" allows you to create the "__doPostBack Script" code which causes a serverside postback. 
  
  
  or example 2) in aspx:
  
            <asp:Button
                ID="btnDataSave"
                runat="server"        
                Text="SAVE"
                OnClientClick='this.disabled=Page_ClientValidate("valgrpData");'
                UseSubmitBehavior="false"
                ToolTip="save data"
                ValidationGroup="valgrpData" />
      
   using this js function:
   
   //check if clientside validation of a single validationgroup is valid
   //returns a boolean value
   function Page_ClientValidate(valGrp)
   {
      Page_InvalidControlToBeFocused = null;
      if (typeof(Page_Validators) == "undefined")
      {
        return true;
      }
      var i;
      for (i = 0; i < Page_Validators.length; i++)
      {
         ValidatorValidate(Page_Validators[i], valGrp, null);
      }
      ValidatorUpdateIsValid();
      ValidationSummaryOnSubmit(valGrp);
      Page_BlockSubmit = !Page_IsValid;
      return Page_IsValid;
   }
      
The button will only be disabled when page validation passed successful.   
   
Problem when using both ways:
In Firefox the disabled button looks the same like an enabled button, in IE it works.
This could irritate the user. Well this is a different behaviour of browsers.
But if necessary there may be more complex ways to solve this problem for example
using different styles or images for the button.

    
Thanxs for Praveen Kumar Battula for his initial idea i found at
http://praveenbattula.blogspot.com/2010/01/disable-button-in-onclick-and-process.html