no duplicates, 'post__not_in' method

<?php
$c = 1; //init counter
$do_not_duplicate = array(); //init array
$bpr = 3; //boxes per row
$my_query = new WP_Query('category_name=Artists&posts_per_page=3&orderby=rand');
while ($my_query->have_posts()) : $my_query->the_post();
$do_not_duplicate[] = $post->ID ?>

<div class="postgrid" id="post-<?php the_ID(); ?>">

<div class="postthumb">
<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_post_thumbnail('grid-post-image'); ?></a><div class="borderthumb"></div><div class="posttitle"><h1><a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a></h1>
  <p><a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">Click for more</a></p></div>
  </div>
  </div>


<?php endwhile; ?>
<div class="clr">
</div>
</div>

<div class="gridContainer">

<?php
$c = 1; //init counter
$bpr = 3; //boxes per row
$my_query = new WP_Query(array('category_name' => 'performance', 'posts_per_page' => 3, 'orderby' => 'rand', 'post__not_in' => $do_not_duplicate));
while ($my_query->have_posts()) : $my_query->the_post(); ?>

<div class="postgrid" id="post-<?php the_ID(); ?>">

<div class="postthumb">
<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_post_thumbnail('grid-post-image'); ?></a><div class="borderthumb"></div><div class="posttitle"><h1><a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a></h1>
   <p><a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">Click for more</a></p></div>
  </div>
</div>

<?php endwhile; ?>