You can do this reasonably reliably client-side with Javascript too...

I've written the below from memory, I have used something similar in the past for exactly this situation...

<script language="javascript">
var submitCount = 0;

function doSubmit()
{
    if (submitCount == 0)
    {
        submitCount++;
        document.form1.submit();
    }
    else
    {
        alert("Please wait, lengthy query executing...");
    }
}
</script>
...
<form name=...>
...
<input type="button" onClick="doSubmit();">
</form>
...
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