Maybe you could use the best of both approaches by presenting the most popular/important/broad tags (e.g. PHP, usability, CPC) as a suggested list while allowing users to add their own, more specific or less popular tags (e.g. drupal, memcached, adsense, appengine).

The problem I see with this is it might not be all that limited. There are quite a few areas of expertise out there, which could make for quite a large form, where as defined "sections" with a free form tag style fill in would make for less clutter. Its "hackers" were talking about after all, most if not all are going to be familiar with a tag system.

As far as the duplicate tags there a number of ways to correct this. First and most prominent is community editing. The other option is to tie tags together with a backend system - for example ruby-on-rails === rubyonrails === rails === RoR