You may well forfit some of the payout. Frankly I'm amazed they paid up without checking. This is one of the things they normally check before paying out. 3 Courses of action: 1. Cash the cheque but ring fence the money and hope they don't push it. 2) iI they do push it then you can try saying you have sent it already and hope their admnistration is an all too common shambles. 3) Come clean and see what they do.
In the case of 3 you may also write separately asking for your previously forwarded documents back, date the letter a week ago and blame it on he post strike!
Before the motoring evangelists start jumping up and down all the above are just ideas I'm not condoning or recommending any of them, merely relaying the possibilities.