Why Is OnBeforeUnload Firing Twice With LinkButton?

I have the following Jquery function to notify the user with a prompt when they have unsaved changes (similar to how SO does it) and are trying to leave the current screen.

<!-- in my aspx page -->
<script type="text/javascript" language="javascript">
window.onbeforeunload = function()
{
if (HasPendingUpdates())
{
return "Changes you have made will not be saved.";
}
}
</script>

This works as expected and shows the message box when the user tries to click a href to browse away or use the back button etc. The problem I am having is that one of my links is a asp:LinkButton because it needs to fire off some server side code before leaving the page. When the user clicks this LinkButton they get the prompt twice.

Scenarios:

User clicks the LinkButton
Prompt appears and user clicks the cancel button.
Prompt disappears and user is still on screen. GOOD.
User clicks the LinkButton
Prompt appears and clicks the OK button.
Prompt disappears and the same prompt shows again.
User clicks OK again.
Prompt disappears and user moves to the next screen and all is good.

So why am I getting the second prompt??? How is the OnBeforeUnload firing twice?