I was seeing something similar when I switched from Wasabi Related Posts to YARPP. Here was the problem:
To call Wasabi, I had to use “echo related_posts()” and the “echo” was causing the list to show up twice with YARPP (I call it manually instead of letting it auto-insert).
The end result looked similar to the problem you’re having, so check your theme and see if this might explain it. Hope this helps.
