Firstly, if people start mirroring your trades, that's a good thing. The extra buying power will help to push the market in your direction.
Secondly, anyone that would be dumb enough to mirror another person's trades is going to go broke. What if the star hedge fund manager is using futures to hedge a share portfolio with a different broker. You copy the futures trades but don't know about the share trades. Suddenly, the you're losing a fortune while the hedge fund manager is market neutral. In practice, it's not very smart.
Secondly, anyone that would be dumb enough to mirror another person's trades is going to go broke. What if the star hedge fund manager is using futures to hedge a share portfolio with a different broker. You copy the futures trades but don't know about the share trades. Suddenly, the you're losing a fortune while the hedge fund manager is market neutral. In practice, it's not very smart.