Many kaggle competitions will have public and private leaderboards. And you are strongly advised to separate out your own validation set from the training data they give you to choose your best model to compare on the public leaderboard. And there are times people have fit to the public leaderboard, but this can be checked with adverserial validation and the like. If you like this kinda stuff, both Abhishek Thakur and Konrad Banachevicz's books are real nice...