Common subdirectories: node_level_blocks/css and node_level_blocks/css
Common subdirectories: node_level_blocks/js and node_level_blocks/js
diff -up node_level_blocks/node_level_blocks.admin.inc node_level_blocks/node_level_blocks.admin.inc
--- node_level_blocks/node_level_blocks.admin.inc	2010-05-19 20:54:26.000000000 +0200
+++ node_level_blocks/node_level_blocks.admin.inc	2011-02-08 15:23:49.000000000 +0100
@@ -40,6 +40,11 @@ function node_level_blocks_node_type_for
     '#title' => t('Display blocks on node-edit pages'),
     '#default_value' => $settings->display_node_edit,
   );
+  $form['node_level_blocks']['node_level_blocks_multi'] = array(
+    '#type' => 'checkbox',
+    '#title' => t('Prevent multi-display of blocks on node'),
+    '#default_value' => $settings->multi,
+  );  
   $form['node_level_blocks']['node_level_blocks_regions'] = array(
     '#type' => 'select',
     '#title' => t('Enabled regions'),
@@ -66,6 +71,7 @@ function node_level_blocks_node_type_for
   $settings = (object) array(
     'enabled' => $form_state['values']['node_level_blocks_enabled'],
     'display_node_edit' => $form_state['values']['node_level_blocks_display_node_edit'],
+    'multi' => $form_state['values']['node_level_blocks_multi'], 
     'regions' => $form_state['values']['node_level_blocks_regions'],
     'modules' => $form_state['values']['node_level_blocks_modules'],
   );
diff -up node_level_blocks/node_level_blocks.module node_level_blocks/node_level_blocks.module
--- node_level_blocks/node_level_blocks.module	2010-05-19 20:54:26.000000000 +0200
+++ node_level_blocks/node_level_blocks.module	2011-02-08 16:51:30.000000000 +0100
@@ -156,14 +156,42 @@ function node_level_blocks_preprocess_pa
         }
       }
       // Theme the block.
-      $regions[$block->region]["$block->module-$block->delta"] = array('#value' => theme('block', $block), '#weight' => $block->weight);
+      $regions[$block->region]["{$block->module}_{$block->delta}"] = array('#value' => theme('block', $block), '#weight' => $block->weight);
     }
-
+    
     // Render the regions.
     foreach ($regions as $region => $data) {
+    
+      // remove duplicated
+      if($vars['node']->node_level_blocks['settings']->multi) {
+    
+        // empty region
+        $vars[$region] = '';
+        
+        // get system block list
+        if ($block_list = block_list($region)) {
+        
+          // remove duplicated
+          foreach ($data as $key => $value) {
+            if(array_key_exists($key, $block_list)) {
+              unset($block_list[$key]);
+            }
+          }
+          
+          // add system blocks in region
+          foreach ($block_list as $key => $block) {
+            $vars[$region] .= theme('block', $block);
+          }          
+         
+        }
+       
+      }   
+     
+      // add node level blocks in region
       $vars[$region] .= drupal_render($data);
       unset($data['#value']);
     }
+    
   }
 }