Indeed, as the post loop loads before the sidebar, ad units contained within the loop display twice or three times, which prevents ad units in sidebar to show up, because of AdSense three ad units per page limitation
This is a simple trick that works with any WordPress theme
Open the index.php file of your theme in your favorite text editor
Before the loop (starting either with foreach or while) add
<?php
$adsense = 'Put_your_AdSense_code_here';
?>
Now, where you want your ad unit to show up, place
<?php
echo $adsense;
$adsense = '';
?>
From now on, your ad unit should show up only on the most recent post and your other ads, that come later in the code, should show up too
Pretty neat, huh?
