I'd like to know some tips you have ever used to protect your videos from re-uploading on Youtube?
And if someone re-uploads a YouTube video then what will happen?
Use Youtube Content ID, it allows you to protect your videos. If someone re-uploads your video on youtube you can choose to either remove that video or monetize it (meaning you can still make money with that video even if it's not on your channel).
ContentID is the best way but you will need some time and effort to enable it on your channel.
Anyway it's easier than finding every single copy of the video and flag it manually.
Furthermore you can get in touch with people who re-upload your videos and ask them for a refund.