It's common for channels to repost old videos so YouTube promotes them as new again. Often with tweaked title and thumbnail to try for more click rates the second time, but sometimes it's just reposting something old enough that due to user turnover it will be new to many of the people it's promoted to.

Also, it's common for channels to steal content from other channels, to slap together with minimal editing what the original creator took weeks to research and design presentation for.