今天在执行 apt-get update 的时候报错，报错内容如下：

W: Duplicate sources.list entry http://extras.ubuntu.com/ubuntu/ precise/main amd64 Packages (/var/lib/apt/lists/extras.ubuntu.com_ubuntu_dists_precise_main_binary-amd64_Packages)
W: Duplicate sources.list entry http://extras.ubuntu.com/ubuntu/ precise/main amd64 Packages (/var/lib/apt/lists/extras.ubuntu.com_ubuntu_dists_precise_main_binary-amd64_Packages)
W: Duplicate sources.list entry http://extras.ubuntu.com/ubuntu/ precise/main i386 Packages (/var/lib/apt/lists/extras.ubuntu.com_ubuntu_dists_precise_main_binary-i386_Packages)
W: Duplicate sources.list entry http://extras.ubuntu.com/ubuntu/ precise/main i386 Packages (/var/lib/apt/lists/extras.ubuntu.com_ubuntu_dists_precise_main_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

在网上搜索后得知，这是因为以上几条，与

deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main

冲突了，于是修改这部分源代码为：

# deb http://security.ubuntu.com/ubuntu quantal-security main restricted
# deb-src http://security.ubuntu.com/ubuntu quantal-security main restricted
deb http://security.ubuntu.com/ubuntu quantal-security universe
deb-src http://security.ubuntu.com/ubuntu quantal-security universe
deb http://security.ubuntu.com/ubuntu quantal-security multiverse
deb-src http://security.ubuntu.com/ubuntu quantal-security multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu quantal partner
# deb-src http://archive.canonical.com/ubuntu quantal partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu quantal main
deb-src http://extras.ubuntu.com/ubuntu quantal main

注释掉两条重复的，于是，ok了