Display warning when admin enters the same discount coupon code

[RomanovM]

nopCommerce version: 4.50

I noticed that i can define 2 discounts with same coupon code.

Source: https://www.nopcommerce.com/boards/topic/94925/duplicate-coupons

See #1462. But we should display a warning (warningValidation) when admin enters the same discount coupon code.

[ahmadkq]

@RomanovM I suggest to prevent having duplicates if there is an active old coupon, otherwise you can allow insert same coupon.

[DmitriyKulagin]

Closed #6353

[AndreiMaz]

@ahmadkq We allow that, it's used in some scenarios. For example, one coupon code for two different discounts (e.g. for some products 20% and for some products 30%). So it's by design