Duplicate posts returned from relationship query

In our product we have custom post types for "products" and "groups", and an mb_relationship "product_to_group"

On the group archive page, we want to include products linked to current group as well as the group's children.

So the archive code first generates an array containing the group id, and the group's children's id's.
$group_array = array(1,2,3,4); // where 1=current id; 2,3,4=children

The we perform our main query:

    $the_query = new WP_Query( array (
        'post_type'      => 'product',
        'post_status'  => 'publish',
        'posts_per_page' => -1,
        'relationship' => array(
            'id' => 'product_to_group',
            'to' => $group_array, 
        ),
    ) );

    if ( $the_query->have_posts() ) {

        while ( $the_query->have_posts() ) : $the_query->the_post(); 

            // Process the post

        endwhile; 

    }

The problem we are seeing is duplicate products. I think what's happening is if a product is in multiple groups, it appears in the results for each matching group.

My workaround will be to use a $used_products array to skip over duplicates, but this feels like a band-aid.

Am I using the plugin incorrectly? Is this a bug / feature?

Thanks