I have the same problem and I've been searchig on the Virtuemart Forum. I found an answer tha coud show us a way to he solution.
"For each Category, your browse page template is attempting to display both the Category's thumbnail image and below that, the Category's name, both together in the same link. It appears you have no Category Thumbnail images, so in place of the image, the browser shows the contents of the img tag's "alt" value, which is the name of the Category, hence you see each Category name twice. To fix, either load images for each Category or modify your browse template to not display the thumbnail image."
I don't know if this is the solution but maybe this is the way.
Another possibility that is happened to me is to modify the file categoryChildlist.tpl.php