Cred Wonderla code says Invalid/Already used

Respected Dimers! While trying to redeem the Cred Wonderla code which I got from Only Fridays offer, it throws me an error coupon code invalid or already claimed but believe me guys I'm 200% sure I haven't used it. Raised this issue with Cred and expecting a resolution by 27th July. My suspicion is all Cred Wonderla codes are the same except the last 4 digits. Can anybody misuse it by randomly changing the 4 digits? Also how long does it take to get the tickets after redemption? Thanks for your answers!