Multiple coupons in Shop Theme

I had some issues with 8.9.4 and coupons - I've done a workaround on that (and will update that thread in case it helps others). However... I have only set up 1 coupon and it works fine i.e. 10% discount. The problem is that if the user tries to add another code OR the same code, the amount changes. Is it possible to force the user to ONLY be able to use 1 coupon? If I remove the products from the basket - the discount remains!! which looks very odd. So my basket has a minus amount in it. Is this a bug? Can it be forced to flush everything out when the basket is empty? By the way, the above is true on a fresh install of 8.9.6 as well as my current 8.9.4 site. What do you think guys? Simon