I promised to let everybody who commented on this article about Medium's no-duplicate know if anybody's comments clarified the policy. So far, nobody's comments have done that, but one writer--you--suggested using a plagiarism checker. This might (or might not) indicate that Medium uses one. The one I'm using checks each sentence for an identical sentence elsewhere. This doesn't tell me what percentage of sentences need to be changed to make the new version an original rather than a duplicate. However, if EVERY sentence is changed, the result surely won't be a duplicate.

If you find out more about this, let us all know.