I think it might be useful as a recommendation for automatic Markdown writers or convertors from other formats: They could generate something very similar to each other, which could likely be a good thing.
Enforced by parsers, imho not so much, if at all. Many apps would likely have to keep some backward compatibility with older documents anyway (and why to keep two so similar parsers around).
Also, I very much doubt that a wide consensus which of the duplicate to keep and which to remove could be reached: Many authors have already chosen whether they prefer e.g. a setext heading over ATX heading or vice versa: If you remove any of the two, the affected people quite likely won’t migrate to such strict parser at all, so it could actually add to the babel instead of solving anything.