Edit masterpage, need to add jQuery - how to ensure it does not duplicate?

SharePoint 2010 Server. I edited section of my masterpage like this(in sharepoint designer, not programmatically):

    <script type="text/javascript" src="http://ift.tt/t0Y3fx"></script> 

But how can I ensure that I don't connect jQuery twice? Something like this

    <script>if (typeof jQuery == 'undefined') { 
    // jQuery is not loaded} 
    else {
    // jQuery is loaded
    }
    </script>

but how can I dynamically load jQuery from here? Or if I edit .masterpage, I should look by myself if there is no jQuery already added?