Post Meta Key sorting – removing duplicates

Hey there, this one’s a challenge. I’m not even quite sure I’m going about this right. I’m trying to build a calendar in WordPress for a client. What I essentially need to do is parse a series of posts for a particular meta key, which contains the date and extrapolate unique dates from those posts. From this loop I should be able to organize the posts into their respective date rows (that part I’ve got figured out).

I hope that makes sense here’s what I have so far, I’m not entirely sure this is even the correct direction:

<?php
$args= array(
                'post_type'=>'cw_events',
                'showposts'=>-1,
                'order'=>'ASC',
            );
            $archive = get_posts( $args );
foreach( $archive as $post ) : setup_postdata($post);

	global $wpdb;

	$metatable = $wpdb->prefix."postmeta";

	$post_id=$post->ID;

	$cal_dates = $wpdb->get_results($wpdb->prepare("SELECT meta_value FROM $metatable where post_id=%d AND meta_key='event_date'",$post_id));

	//output posts -- trying to use array_unique(), but it's not working

	$cal_dates = array_unique($cal_dates, SORT_REGULAR);

		foreach ($cal_dates as $cal_date) {

		    echo var_dump($cal_date);
		    echo $cal_date->meta_value.'<br />';

		}

endforeach; ?>

So far it just outputs all of the data from each of the posts without parsing it for duplicates… Not sure what to do.

Thanks for the help in advance!