my up date-manager is not running

my system used to update quite often. but for months now there has not been any update at all. i tried opening the update manager by clicking on it and this is what it shows: COULD NOT INITIALIZE THE PACKAGE INFORMATION.Please report this bug against the 'update-manager' package and include the following error message:

'W:Duplicate sources.list entry http://ng.archive.ubuntu.com/ubuntu/ natty/main i386 Packages (/var/lib/apt/lists/ng.archive.ubuntu.com_ubuntu_dists_natty_main_binary-i386_Packages), W:Duplicate sources.list entry http://ng.archive.ubuntu.com/ubuntu/ natty/restricted i386 Packages (/var/lib/apt/lists/ng.archive.ubuntu.com_ubuntu_dists_natty_restricted_binary-i386_Packages), W:Duplicate sources.list entry http://ng.archive.ubuntu.com/ubuntu/ natty-updates/main i386 Packages (/var/lib/apt/lists/ng.archive.ubuntu.com_ubuntu_dists_natty-updates_main_binary-i386_Packages), W:Duplicate sources.list entry http://ng.archive.ubuntu.com/ubuntu/ natty-updates/restricted i386 Packages (/var/lib/apt/lists/ng.archive.ubuntu.com_ubuntu_dists_natty-updates_restricted_binary-i386_Packages), E:Encountered a section with no Package: header, E:Problem with MergeList /var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_natty-security_restricted_binary-i386_Packages, E:The package lists or status file could not be parsed or opened.'