Cannot redeclare pclziputilpathreduction() upgrade error and how to fix it!

I have a blog that wasn’t update for a very long time. Its last post was March 25, 2009. I’m still using WordPress 2.7 in that blog. After attempting to upgrade it to WordPress 3.2, I encountered the error: Cannot redeclare pclziputilpathreduction().

Now I wonder why I cannot update it. I tried figuring it by disabling my current WordPress plugins one by one, until I disabled WPAU. WPAU is the WordPress Automatic Upgrade plugin which is completely useless in the newer version of WordPress. Why? Because the WordPress core itself has an automatic upgrade feature and you don’t need to use this plugin to upgrade your blog to the latest WP version.