The technology to prevent that without blockchains is there. It is standard for online banking, credit cards and even small online shops. Money duplication is pretty much non-existing. The only way to "cheat" here is to hack accounts and steal existing money (unless you are a bank that creates checkbook money, but that is another story). If MMOs have problems with exploits than not by missing technology but by bad design and programming. Block chains without proper implementation wouldn't solve this problem. And whoever can implement them properly would also be able to do it the traditional way, which is much faster and cheaper.