Skins in Minecraft aren’t private, and as a server we have no way of knowing who originally made or owns a specific skin—or if multiple players are using the same one.
It’s possible for players to change their skin using certain tools or methods, but from our side, it just looks like they’ve changed their skin normally. Because of that, we’re not able to tell if someone is copying a skin or prevent it from happening.
We get that it can be annoying, but wearing the same skin as someone else doesn’t break any rules. If the player is doing anything else that seems inappropriate, you can always report them in-game using the /report command.