Show a message to users that have purchased the currently viewed product in EDD.

gistfile1.php

<?php
/**
 * Show Has Purchased Item Message
 *
 * Prints a notice when user has already purchased the item.
 *
 * @since 1.0
 * @global $user_ID
 * @param int $download_id Download ID
 * @return void
 */
function pw_edd_show_has_purchased_item_message() {
	global $user_ID, $post;

	if( !isset( $post->ID ) )
		return;

	if ( edd_has_user_purchased( $user_ID, $post->ID ) ) {
		$alert = '<p class="edd_has_purchased">' . __( 'You have already purchased this item, but you may purchase it again.', 'edd' ) . '</p>';
		echo apply_filters( 'pw_edd_show_has_purchased_item_message', $alert );
	}
}
add_action( 'edd_after_download_content', 'edd_show_has_purchased_item_message' );