Duplicate images with image placeholder services?

I have a little problem with duplicate images filled by placeholder services.

I have random php array like this:

        $images = array(
            'http://ift.tt/1CC41dm',
            'http://ift.tt/1aQFmwh',
            'http://ift.tt/1CC41dq',
            'http://ift.tt/1CC41ds',
            'http://ift.tt/1aQFmwh',
            'http://ift.tt/1aQFmwp',
            'http://ift.tt/1hDdf3G',
        );
        $output = $images[array_rand($images)];

The problem is: if i use this to fill 20 placeholders on one page then there are multiple image duplicates, every image is used at least twice. Shouldnt these placeholders load random images out of tens or thousands of them? Or what am i doing wrong?