How can I tell whether a coupon that I received by post is the same one that I activated and redeemed digitally the day before?

The coupons that you have already redeemed are listed in the coupon portal under "Redeemed". You’ll find a seven-digit coupon number (C-ID) on the paper and digital coupons in the detail view. If the number on the paper coupon is the same as that on the redeemed digital one, you can no longer redeem the paper coupon.