Duplicate Submit Button Clicks

Hi, I have created a web form for staff to enter information into which is stored in SQL. The page has a button:

<asp:Button ID="Button1" runat="server" Text="Submit Request" />

And in the code behind I have a sub:

 Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click

Which adds the data to the database and sends off some confirmation emails.

These tasks can take about 5 seconds to complete, and what happens is the user thinks they haven't clicked the button and clicks it again (causing a duplication of information).

Is there a way to stop them being able to submit the information twice?

We have let people know not to click the button twice but people still do!

Many Thanks