Artist name the same,,,, but differant?

sometimes when i tag or retag a song i get an artists name listed twice as though they are differant artists. this is almost impossable to explain buti download a song from an artists i already have on my ipod but the ipod would lidt the artist twice as though the song i downloaded is a differant srtist that i have on my ipod. you understand? i have to go and retag ALL the songs by the artist to make the ipod recgognize them as the same. why does that happen? cant itunes and the ipod recgognze its its spelled the exact same way it is the same? why not??

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