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There are two possibilities I can think of as to why your iTunes icon is different.

1. You've installed a theme and some theming software that changes the iTunes icon.
2. You used a dock color changing program and iTunes somehow got a custom icon (you can do this to any file by selecting it, doing a Get Info on it, selecting the icon, and pasting any picture on top of it - this includes the iTunes application).
 
Have you tried doing a 'get info' on the app in the apps folder, selecting the icon at the top of the info window and pressing delete?
 
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