Prevent user from clicking submit multiple times

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ASP.NET

I have disabled the Submit button on my web page to prevent users from submitting it multiple times. I want to save user data only once into the database when user hit the submit button.
Most of the time it works fine but sometime I ended up having multiple entries of same records in database. Is there anything that I am doing wrong here ? Any other suggestion ?

StringBuilder sbValid = new StringBuilder();

            sbValid.Append("if (typeof(Page_ClientValidate) == 'function') { ");

            sbValid.Append("if (Page_ClientValidate('RequiredFields') == false) { return false; }} ");

            sbValid.Append("this.value = 'Please wait...';");

            sbValid.Append("this.disabled = true;");


            //GetPostBackEventReference obtains a reference to a client-side script function that causes the server to post back to the page.

            sbValid.Append(this.Page.GetPostBackEventReference(this.btnSubmit));

            sbValid.Append(";");

            this.btnSubmit.Attributes.Add("onclick", sbValid.ToString());

RequiredFields is the name of validation control group on that page.

The worst thing about this situation is I cannot recreate the issue. It is very random. :(