Some of the challenges from RTLM or the new Marquis World Championship need us to finish at the second place (silver medal) or the third place (bronze medal). Thats mean there will be silver or bronze medals permanently stay on our statistic record.
I found there are two ways to avoid this condition.

First, use your GC to skip the challenge, and i think this is the easiest and the best way.. lol.. ^_^

Second, finish the challenge as it ask to complete the challenge (finish at the 2nd or 3rd).
When you have completed the goal, don't click continue button, but click retry button and race again but make sure you finish at the first place so you got the gold medal.
Make sure your car is at the good condition, so when you retry to race at the second time to get the gold medal, your car still can compete well.
I've tried it by myself and its work.

So, which step do you like?