apt-get update aborts with errors

["jose isaias cabrera" <jicman@frontiernet.net>]

Happy Festivus and happy New Years Eve!

I have ran

apt-get update

command and I am getting this error:
...
...
Hit http://ftp.us.debian.org stable/main Release
Fetched 5342kB in 60s (87.7kB/s)
Reading Package Lists... Done
Building Dependency Tree... Done
W: Duplicate sources.list entry http://ftp.us.debian.org stable/main 
Packages 
(/var/lib/apt/lists/ftp.us.debian.org_debian_dists_stable_main_binary-i386_Packages)
W: Duplicate sources.list entry http://non-us.debian.org stable/non-US/main 
Packages 
(/var/lib/apt/lists/non-us.debian.org_debian-non-US_dists_stable_non-US_main_binary-i386_Packages)
W: You may want to run apt-get update to correct these problems

Of course, after running the 'apt-get update' command, as the error 
suggested, I get the same message.  I did just run it again and it fixed it. 
Any idea how this happens?

thanks.