It looks like someone installed the extra definitions file for the AME modification. This created a double entry for youtube. The double entry needs to be removed.
What having a double entry for youtube does - it embeds the video twice. I just posted a thread about a new boat, posted 2 videos in the thread, and each video was embedded twice.
The same thing happened on my forum when I installed the extra definitions file. Someone needs to go into the AME definitions, and remove one of the youtube entries - so that there is only one.
If you look at the AME definitions, you might find doubles for youtube, google video, dailymotion, and maybe metacafe. At least that is what happened on my forum.