Comment 17 for bug 1558331

I can confirm that I've been seeing this the past couple of days too:
W: gpgv:/var/lib/apt/lists/ppa.launchpad.net_noobslab_themes_ubuntu_dists_xenial_InRelease: The repository is insufficiently signed by key 4FA44A478284A18C1BA4A9CAD530E028F59EAE4D (weak digest)
W: gpgv:/var/lib/apt/lists/ppa.launchpad.net_teejee2008_ppa_ubuntu_dists_xenial_InRelease: The repository is insufficiently signed by key 1B32B87ABAEE357218F6B48CB5B116B72D0F61F0 (weak digest)
W: gpgv:/var/lib/apt/lists/ppa.launchpad.net_webupd8team_java_ubuntu_dists_xenial_InRelease: The repository is insufficiently signed by key 7B2C3B0889BF5709A105D03AC2518248EEA14886 (weak digest)
W: gpgv:/var/lib/apt/lists/deb.opera.com_opera-stable_dists_stable_InRelease: The repository is insufficiently signed by key 419D0ACF314E8E993F7F92E563F7D4AFF6D61D45 (weak digest)

I do know that these are just warnings, but will confuse some people.