how to stop body onload= event from firing when user clicks server side button

Hi ,
how to stop body onload=  event from firing when user clicks server side button .
this is my below code in client side
<head>
<script type="text/javascript" language="javascript">

function addpopup()
{
window.open('Ship_Complete.aspx','window','width=600px,height=300px,left=600px,top=300px','_blank');
return false;
}
 
</script>
</head>
 
<body onload ="addpopup();" >

when i clicks server side button it raises body onload function again?
I want to raise this body onload function only when page load not in server side button click.
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