rel="dofollow" and rel="follow"

I've been getting a fair number of blog comments lately with link attributes like rel="dofollow" and rel="follow". Their purpose is to try to override the CMS' nofollow settings. Trying to override my website settings is extremely obnoxious. I immediately ban anyone who leaves comments like that.

I don't believe that search engines will start following links that have those made-up attributes on them, though it's an unknown because technically, an HTML element can't have two rel attributes. Do any search engines discard the 2nd attribute? (nofollow)

<a href="http://www.example.com" rel="dofollow" rel="nofollow">link spam</a>

It would be great to hear from the search engines about whether they discard a second rel attribute when they find it.

Example — does a fake rel attribute before the nofollow cause bots to ignore the rel attribute after it:

<a href="http://www.example.com" rel="1234" rel="nofollow">link spam</a>