"From Wikipedia's article on Mario:
"Mario's full name is believed to be Mario Mario, while his brother's full name is believed to be Luigi Mario, thus making them the "Mario Brothers". (This is generally accepted as canon. According to Shigeru Miyamoto, the Mario Bros. movie, and the television series, their surname is Mario, but this has never been officially supported by Nintendo.)""
That's exactly the reason why I'm bringing it up. Because not everyone believes their last name can be Mario.