Anche io ogni tanto ho lo stesso problema. Vi posto quello che mi è venuto fuori dal terminale:
lo@lo-desktop:~$ sudo apt-get update
[sudo] password for lo:
Ign
http://download.tuxfamily.org gutsy Release.gpg
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http://it.archive.ubuntu.com karmic Release.gpg
Ign
http://download.tuxfamily.org gutsy/avant-window-navigator Translation-it
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http://security.ubuntu.com karmic-security Release.gpg
Ign
http://security.ubuntu.com karmic-security/main Translation-it
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http://it.archive.ubuntu.com karmic/main Translation-it
Ign
http://download.tuxfamily.org gutsy Release
Ign
http://security.ubuntu.com karmic-security/restricted Translation-it
Ign
http://security.ubuntu.com karmic-security/universe Translation-it
Ign
http://security.ubuntu.com karmic-security/multiverse Translation-it
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http://security.ubuntu.com karmic-security Release
Ign
http://download.tuxfamily.org gutsy/avant-window-navigator Packages
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http://it.archive.ubuntu.com karmic/restricted Translation-it
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http://security.ubuntu.com karmic-security/main Packages
Ign
http://download.tuxfamily.org gutsy/avant-window-navigator Packages
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http://it.archive.ubuntu.com karmic/universe Translation-it
Err
http://download.tuxfamily.org gutsy/avant-window-navigator Packages
404 Not Found
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http://security.ubuntu.com karmic-security/restricted Packages
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http://security.ubuntu.com karmic-security/main Sources
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http://security.ubuntu.com karmic-security/restricted Sources
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http://security.ubuntu.com karmic-security/universe Packages
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http://it.archive.ubuntu.com karmic/multiverse Translation-it
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http://security.ubuntu.com karmic-security/universe Sources
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http://security.ubuntu.com karmic-security/multiverse Packages
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http://security.ubuntu.com karmic-security/multiverse Sources
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http://it.archive.ubuntu.com karmic-updates Release.gpg
Ign
http://it.archive.ubuntu.com karmic-updates/main Translation-it
Ign
http://it.archive.ubuntu.com karmic-updates/restricted Translation-it
Ign
http://it.archive.ubuntu.com karmic-updates/universe Translation-it
Ign
http://it.archive.ubuntu.com karmic-updates/multiverse Translation-it
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http://it.archive.ubuntu.com karmic-proposed Release.gpg
Ign
http://it.archive.ubuntu.com karmic-proposed/restricted Translation-it
Ign
http://it.archive.ubuntu.com karmic-proposed/main Translation-it
Ign
http://it.archive.ubuntu.com karmic-proposed/multiverse Translation-it
Ign
http://it.archive.ubuntu.com karmic-proposed/universe Translation-it
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http://it.archive.ubuntu.com karmic Release
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http://it.archive.ubuntu.com karmic-updates Release
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http://it.archive.ubuntu.com karmic-proposed Release
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http://it.archive.ubuntu.com karmic/main Packages
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http://it.archive.ubuntu.com karmic/restricted Packages
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http://it.archive.ubuntu.com karmic/main Sources
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http://it.archive.ubuntu.com karmic/restricted Sources
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http://it.archive.ubuntu.com karmic/universe Packages
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http://it.archive.ubuntu.com karmic/universe Sources
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http://it.archive.ubuntu.com karmic/multiverse Packages
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http://it.archive.ubuntu.com karmic/multiverse Sources
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http://it.archive.ubuntu.com karmic-updates/main Packages
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http://it.archive.ubuntu.com karmic-updates/restricted Packages
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http://it.archive.ubuntu.com karmic-updates/main Sources
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http://it.archive.ubuntu.com karmic-updates/restricted Sources
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http://it.archive.ubuntu.com karmic-updates/universe Packages
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http://it.archive.ubuntu.com karmic-updates/universe Sources
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http://it.archive.ubuntu.com karmic-updates/multiverse Packages
Trovato
http://it.archive.ubuntu.com karmic-updates/multiverse Sources
Trovato
http://it.archive.ubuntu.com karmic-proposed/restricted Packages
Trovato
http://it.archive.ubuntu.com karmic-proposed/main Packages
Trovato
http://it.archive.ubuntu.com karmic-proposed/multiverse Packages
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http://it.archive.ubuntu.com karmic-proposed/universe Packages
W: Impossibile recuperare
http://download.tuxfamily.org/syzygy42/ ... es.gz 404 Not Found
E' sto ultimo indirizzo internet che mi da dei problemi!