Make it an error to attach a plugin multiple times
Make it an error to attach a plugin multiple times

To avoid problems with implicitly overriding options

salzhrani/prosemirror,salzhrani/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,salzhrani/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror