Shopify "Add to Cart" Buttons adding 2 Items instead of 1

[sandiaseed]

Watch this video to see the issue: 
https://www.youtube.com/watch?v=DY9RSeDTj70

Our Shopify "Add to Cart" Buttons on product pages on sandiaseed.com is adding 2 Items instead of 1 – randomly – to the cart.  See how inconsistent this is, sometimes a product's Add to Cart button will add one item to the cart, but sometimes it will add 2 even though only 1 qty is selected. It's never the same, and very random. The quantity glitch is inconsistent even on the same product it can do this as seen in the above video. 

Does anyone have any ideas on what might be causing this?
It seems to be a new problem, but we were only recently made aware of it so it may have been occurring for a long time.

[LitExtension · Accepted solution]

Hi @sandiaseed,

I checked and your site link is not showing like the video. 

And after checking the error on the video, this is most likely to happen when you install the add to cart app and it generates a double-click about submit.

Can you send me the site link, I will check it.

[sandiaseed]

We upgraded our theme, which seemed to be causing the issue, and this issue resolved.