BuddyPress.org

I’m having the same problem on Buddypress 1.8.1 and latest WP 3.6. I am using Suffusion as my theme. The Leave Group button is appearing twice on the Groups list page on each group description.
To fix this I went into my child theme folder and edited groups/groups-loop.php

I deleted the following:

Line 40 <?php bp_group_join_button() ?>

Line 46 <?php do_action( ‘bp_directory_groups_actions’ ) ?>

If you do this then users will not be able to join groups from the Groups list page but will have to click on the link for a group and then click the Join Group button from the Group description page. If you want to keep a Join/Leave group button on the Group list page then you will have to experiment with leaving line 46 most likely.