From OrganicDesign Wiki
# This allows JS from all extensions to be included only once
if (!is_array($wgJavaScriptFunctions)) {
$wgJavaScriptFunctions = array();
$wgHooks['OutputPageBeforeHTML'][] = 'wfAddJavaScript';
function wfAddJavaScript() {
global $wgOut,$wgJavaScriptFunctions;
$script = '<script language="javascript">';
foreach ($wgJavaScriptFunctions as $k => $js) $script .= "\n$js\n";
$script .= '</script>';
$wgOut->addScript($script);
return true;
}
}