不要回显先前值中记录的项目

Hi i have a problem retriving items from my database, I want to retrieve the data just one time and if an item was already retrieved in a previous value to not show that item

PHP

$select_query = "select * from posts";

$run_query = mysql_query($select_query);
while($row=mysql_fetch_array($run_query)){
$post_id = $row['post_id'];
$post_title = $row['post_title'];
$post_date = $row['post_date'];
$post_author = $row['post_author'];
$post_image = $row['post_image'];
$post_keywords = $row['post_keywords'];
$post_content = $row['post_content'];

HTML

<h1 class="centru">The Posts Author are: <?php echo $post_author; ?>
</h1>

EX: On the page this will be displayed:
Jhon
Maria
Jhon
Alex
Jhon
I want to be like this
Jhon
Maria
Alex

If you just want authors, use

SELECT DISTINCT post_author FROM posts

Use GROUP BY functions!

$select_query = "select * from `posts` group by `post_author`";