Add option for stacking discountCodeRule with itself

The customer creates a discountCodeRule that is stackable, but they don’t want it to be stackable with itself. Today, you can have multiple discount codes connected to it, and all of them are possible to apply to the same session. This isn’t necessarily something you want if you, for example use generic influencer codes like “Elin20” and “Sofia20” to apply a 20% discount on the session.