Products Getting Duplicated In Customer's Cart

[DuaFatimah26]

Hey guys, so we recently launched a website that has fully personalised products and an issue keeps arising for only some of our customers and some of our team members where whenever they are finished designing the product and add it to their cart then it automatically creates a duplicate of the exact same design in the cart. This is causing quite a bit of confusion as it doesn't always occur and customers abandon their designs/orders out of confusion. 

I've already spoken to the theme creators and the app we're using but they both keep blaming each other or haven't been able to find a solution either 😞

Theme being used: Highlight By KrownThemes
App being used: Posterly By Shoptigrator

Website: http://giftfootprints.com

Any help would be really appreciated - thanks!