Look, that's a great proof and all but it has nothing to do with the situation at hand. Just because mathematics say there can be two maximal elements doesn't mean morality agrees - and this is the domain of morality, not mathematics.
I disagree. When you say “I just had the most exhilarating experience”, that doesn’t mean that there can only be a single most exhilarating experience. And, independent of that, morality also doesn’t imply that there can be only one most fundamental duty.
Ridiculous. Stop playing pointless language games.