I have an issue where scripts are loading multiple times causing the Welcome menu (by the users name in the header) to have multiple references "My profile" and "My Site"
I have the issue on both the custom master page and the unmodified v4.master inspecting with firebug reveals SP.UI.MyLinksRibbon.js being loaded dynamically multiple times.
<script type="text/javascript" src="/_layouts/SP.UI.MyLinksRibbon.js"></script>
<script type="text/javascript" src="/_layouts/SP.UI.MyLinksRibbon.js"></script>