Initially we all had just one Mac and one iPod. We’d dutifully sync our iPod with our Mac, and life was grand.
Then we got a MacBook, another iPod, and finally, an iPhone. Soon it
was apparent that our music library was scattered across multiple
machines and the idea of trying to figure out which machine had which
songs became an onerous task. So we just threw them all together,
resulting in tons of unwanted duplicate tracks.