I'm still getting theses lines in the top of my page source, but I'm not using the admin front end. Could I have forgotten to remove something from my theme files?

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It's in all of my pages: http://www.jacktummers.nl/collectie/fotografie/

I don't see it. Are you sure you're not logged in ?

Those lines will show up only if you are logged in. They are the messages for the javascript confirmations in the admin toolbox.