As for material vetting, in past years, what usually happens is that some igmc vigilante (AKA people jealous that you won) will go to great lengths to expose you, posting evidence of your prior work in forums so the judges will disqualify you. Believe me there’s always someone. It works surprisingly well. So to save the hassle, just use legit stuff and you have nothing to worry about.
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We do try to check as thoroughly as we can on the winning entries, but it is hard to catch everything, so we do rely at least some on other users to find things like this.
Honestly, I just wish everyone would just follow the rules so everyone has an equal chance, but you know, people are people unfortunately.