la libreria stessa va inclusa una volta sola, tu la includi 3 volte

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.0/jquery.min.js"></script>
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<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.6/jquery.min.js"></script>
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<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.3/jquery.min.js"></script>