O.k. solução rápida para você:
sudo su
rm /etc/apt/sources.list.d/canonical_partner.list
rm /etc/apt/sources.list.d/canonical_partner.list.save
O próximo bloco que você pode copiar e colar completamente no terminal e pressionar enter uma vez:
cat > /etc/apt/sources.list << EOF
# deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
deb http://archive.ubuntu.com/ubuntu xenial main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu xenial-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu xenial-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial-backports main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu xenial-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu xenial-security main restricted universe multiverse
deb http://archive.canonical.com/ubuntu xenial partner
deb-src http://archive.canonical.com/ubuntu xenial partner
EOF
exit
Depois disso, basta fazer:
sudo apt-get update
sudo apt-get dist-upgrade
E tenho certeza de que suas mensagens de entrada dupla desapareceram.