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If the external site links to what Google considers a 'bad neighbourhood', then yes, the external site linking to the doorway page might get penalised but it isn't normally an instant penalty, but after Google figures it out through their various algorithms.
<link rel="canonical" href="<a class="external" rel="nofollow" target="_blank">http://www.sitename.com/</a>" />
would be correct. Don't forget the double quotes!