It isn't ignored.  The first definition is entered into the class dictionary, then the second definition replaces it.  That's why Victor talked about redefining a method, because that's what happens.  You can't really disentangle the two cases except by source inspection (a linter).  In the parameter list case, the parser has the information it needs to detect it in hand, and more importantly there is no meaning to "replacing" a parameter, so we can and do generate an error in that case.