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Anytime you see a file with the .old suffix, it means it's not in use anymore but is there to fallback on should there be a problem. Is it definitely a hyphen after passwd/shadow? If it's a tilde (shadow~) that means the file has been edited by hand.
If you have not edited these files yourself and your system isn't an upgrade from an earlier release I'd recommend using a rootkit scanner to ensure you haven't been compromised.
The one with the tilde is created automatically when you make a manual change with an editor, such as emacs. The .OLD file was probably made when you installed or configured something that modified the original file.
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