使用mongodb的group方法，为什么每次刷新页面，数据会增长

[我听闻始终一个人]

官方拷贝过来的源码
$collection=$mongo->user->food;
$collection -> insert (array( "category" => "fruit" , "name" => "apple" ));
$collection -> insert (array( "category" => "fruit" , "name" => "mangos" ));
$collection -> insert (array( "category" => "fruit" , "name" => "banana" ));
$collection -> insert (array( "category" => "veggie" , "name" => "corn" ));
$collection -> insert (array( "category" => "veggie" , "name" => "broccoli" ));
$keys = array( "category" => 1 );
$initial = array( "items" => array());
$reduce = "function (obj, prev) { prev.items.push(obj.name); }" ;
$g = $collection -> group ( $keys , $initial , $reduce );
echo json_encode ( $g [ 'retval' ]);
第一次查看网页。结果如下
//[{"category":"fruit","items":["apple","peach","banana"]},{"category":"veggie","items":["corn","broccoli"]}]
但是随着我不停刷新页面，页面会数据会增长。
仔细查看了下增长的代码，每次会加每组的数据，比如第一次刷新，增加fruit下的数据，第二次刷新增加veggie下的数据，一直这样循环增长下去。
请问如何停止这样的增长，让数据第一次