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I'm on Ubuntu 16.04 and i get this warning :

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W: http://download.mono-project.com/repo/debian/dists/wheezy/snapshots/3.12.0/InRelease: Signature by key 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF uses weak digest algorithm (SHA1)

just after the :

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sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb http://download.mono-project.com/repo/debian wheezy/snapshots/3.12.0 main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
sudo apt-get update

Anyone know how could i fix this ? plz