After you have enable the Product attribute module, you could adjust the SKU for products having different attributes. But there is no validation to check the SKU uniqueness. I try to enforce the checking by creating a custom module.

custom.module

function custom_form_alter(&$form, &$form_state, $form_id) {
  // Unique SKU check
  if ($form_id == 'uc_product_adjustments_form') {
    $form['#validate'][] = 'unique_sku_validate';
  }
}

function unique_sku_validate($form, $form_state) {
  //print_r($form_state);
  
  // Get the current SKUs
  $current_skus = array();
  $this_skus = array();
  $sql = "SELECT nid, model FROM uc_product_adjustments";
  $result = db_query($sql);
  while ($row = db_fetch_object($result)) {
    if ($row->nid != $form_state['values']['nid']) {
      $current_skus[] = $row->model;
    } else {
      $this_skus[] = $row->model;
    }
  }
  
  $message = "Some SKUs already exists:";
  $has_error = FALSE;
  $user_inputs = array();
  foreach($form_state['values']['body'] as $sku) {
    if (in_array($sku['model'], $current_skus)) {
      $message .= $sku['model'] . '; ';
      $user_inputs[] = $sku['model'];
      $has_error = TRUE;
    }
  }

  // Show error if SKUs already exist in DB
  if ($has_error) {
    form_set_error('body', $message);
  }
  
  // Show error if the user inputs have duplicated entries
  if (has_dupes($this_skus)) { 
    form_set_error('body', "Duplicated entries.");
  }
}

function has_dupes($array) {
  $dupe_array = array();
  foreach($array as $val){
    if(++$dupe_array[$val] > 1){
      return true;
    }
  }
  return false;
}

Done =)