Fix bug dispatching by dublicate plugins#353

Dotbot performs some monkeypatch during loading, where it scans plugin modules and collects every class that inherits from the `Plugin` interface. This makes it easy to collect the same class objects. Plugins such as [if](https://github.com/wonderbeyond/dotbot-if), [ifarch](https://github.com/ryansch/dotbot-ifarch) and [ifplatform](https://github.com/johnlettman/dotbot-ifhostname) contain the following line of code: `from dotbot.plugins import Clean, Create, Link, Shell` This line is sufficient to collect duplicate plugins and dispatch the same configuration instructions twice. See: https://github.com/ssbanerje/dotbot-ifplatform/pull/4#issuecomment-1532112176 https://github.com/anishathalye/dotbot/pull/351