how to prevent duplicates in dynamic drop down list

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SQL

PHP

below is the code FORM with dynamic drop down list

PHP

<?php
$select_query=          "Select category from books";
$select_query_run =     mysql_query($select_query);
$select_query_array=   mysql_fetch_array($select_query_run) ;
 $cat = $select_query_array['category'];
echo "<select name='category'>";
while ($select_query_array=   mysql_fetch_array($select_query_run) )
{
  echo "<option value='' >".htmlspecialchars($select_query_array["category"])."</option>";
}

echo "</select>";
?>

under category column i have different types of books categories like "fiction,journal,non-fiction, etc". to show this I created dynamic drop down list which will fetch all values under category column from database table. my problem is I am unable to prevent duplication for this. ex: category have four fiction and two journal. my drop down list is also showing four fiction and two journals. I don't want to show like this way for any values I want to show only fiction,journal,non-fiction.
simply I want to prevent duplication is drop down list. please anyone help me....