It does not play "twice as much" of the 1UP sound. It plays some of the tail wagging sound slowed down, and then the 1UP sound, one after the other. Also, people write in easter eggs all the time. Like Totaka's song. Not to mention that it's a lot easier to leave random bits of programming in the ROM than clean it before release, since the space is going to be on the chip either way. (That is not the reason for this glitch. However, it is/was frequently done.)