The development process is pretty awkward at the moment, sorry. I've got several ideas to improve it such as hosting a plugin on a local webserver rather than having to install it to browser storage constantly.
Currently if you get an error like that you just have to clear browser storage. Open dev tools, go to the Application tab, click the "Clear storage" category, then click "Clear site data". When it's done, reload. Your plugin will be uninstalled and you can try again.