User must click Back button twice after adding item in cart
While, not an specific answer. Just to point out another possibly cause, this could be caused by JS.

https://developer.mozilla.org/en-US/docs/Web/API/History_API#Adding_and_modifying_history_entries

Basically, there are ways of modifying the history via JS. There might be some code somewhere that's introducing this. Not sure which module though. 

Assuming JS, what you probably should look into is calls to,

 history.pushState() 
 history.replaceState()