A typical scenario for this double discounting is a person comes in a shop and buys an item that you have on sale. He then shows that he is on one of loyalty programs you support so giving him what is called a double discount.
To prevent this happening, please do the following.
Go to the system maintenance, marked with the red arrow.
Then find this option "prevent discounting of sale items," marked with the red arrow here.
Set this to yes as I have here.
This will stop the double discount.
