For anyone having issues with Brothers competition site I got following feedback:
There are certain scenarios with browser where maybe a double click causes the problem although in most cases there is a block on a second click. The first click is the successful entry while the second is trying to enter again but is so soon after that the user doesn't see the message yet and is told the code has already been used... Which it has by that user less then a second earlier.
We are monitoring this and will look to solve this unusual scenario but apologies for now.
If you enter again, please make sure to single click the entry button.