Not sure if you've resolved this yet. I stumbled onto your post while searching for the answer myself. My problem was not specifically jcomments related, as it appeared on every comment extension I used. After not finding a quick solution online and seeing that you dealt with the same issue (on a Rocket Theme, just like me) I decided to check out the default.php for the rocket theme's com_content>article section. I found that they call the same line of code two times. I commented out one of these and it resolved the issue. NOTE: I have no idea if this was intentional or not. I just implemented the solution a few hours ago and have not noticed any ill effects so far. But if you have access to the rocket theme forums you may want to go on there and ask about it to be safe.
Here was the server path to the file that I modified:
The same code was called twice, on line's 224 and 231. I commented out the second entry that was on line 231 for me (it may be different for you). If it's not on the same line just search for the phrase "afterDisplayContent" and look for the line below. If you don't see the line in there 2 times- do not comment out! Your problem is probably something else.
<?php echo $this->item->event->afterDisplayContent; ?>
I changed it to:
<?php //echo $this->item->event->afterDisplayContent; ?>
That solved the issue for me. Hope this helps.