It's not coming from anywhere, it's actually in your post content itself.
Here's a single post with the problem:
Here it is on your main page. Note that the Continue Reading link ends and has no extra /a:
Now, looking at it in the RSS feed (which simply removes the <!--more--> tag entirely, you see that the /a is actually in there:
Edit the post. View the code. The extra /a is actually in the code, I think. Or it's being added after that more link in the editor, somehow.
You could try turning off the visual editor to see what's really in there without the prettified editor obscuring it from you.