How Facebook.com knows about duplicate pages on my site?

Question

I have facebook Like Button and I use Wordpress 3.4.1. I notice if I make "like" on URL like http://example.com/{post_type}/{post_slug}/ facebook knows about copy of this page on URL http://example.com/?{post_type}={post_slug}. I see that Like Button is on and off on these pages simultaneously. And I don't understand why. I've disabled <link rel="canonical"> tag. I've looked for HTTP-headers like "Link" or "Canonical" or something else on my pages and I have had nothing about it. I have no Open Graph Protocol on my site. But facebook.com knows that these pages are just duplicates. Where is the answer?

Answer 1

They are the same resource.

The actual URL is http://example.com/?{post_type}={post_slug}

The rewrite URL (via permalink settings/.htaccess) is http://example.com/{post_type}/{post_slug}/

Answer 2

I've disabled link rel="canonical" tag.

Why? Re-enable that and your duplicate problems will likely go away. Facebook and other crawlers read that tag to know what the correct URL is and to eliminate duplicate content penalties.