I have a fresh install of Opencart 1.5.6.4 - link is here
On all my product pages (not the category pages) - when you click 'add to cart' it adds x2 of everything.
I have tried re-creating a product with a new SEO title, title, product, new model number - but still, it adds it twice to my cart?
Any ideas? Any help would be greatly appreciated.
This is definitely a conflict or bug with the mmenu jquery extension being used for your nav bar and being included as jquery.mmenu.min.all.js.
Comment out the line:
$('nav#menu').mmenu();
And you'll see the cart button functions normally. Unfortunately this breaks your nav bar. I wish I could tell you more but perhaps worth contacting the developer of that extension:
http://mmenu.frebsite.nl/support/problem-solving.html
or file an issue here:
https://github.com/BeSite/jQuery.mmenu/issues
Since you appear to be running the most recent version, if there is indeed a bug it hasn't been fixed yet. I've gone ahead and added the mmenu tag to your question which hopefully may draw the attention of someone with answers. Good luck.