I tried updating through apt-get update and been having these errors. This is a fairly new system and used v158 strech image.
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DietPi | 12:43 | Tue 02/01/18
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V159 | RPi 3 Model B (armv7l)
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root@DietPi:~# apt-get update
Get:1 http://download.mono-project.com/repo/debian stretch InRelease [5,860 B]
Ign:1 http://download.mono-project.com/repo/debian stretch InRelease
Hit:2 http://dev2day.de/pms stretch InRelease
Hit:3 https://archive.raspberrypi.org/debian stretch InRelease
Get:4 https://apt.sonarr.tv master InRelease [13.3 kB]
Ign:4 https://apt.sonarr.tv master InRelease
Get:5 https://www.mirrorservice.org/sites/archive.raspbian.org/raspbian stretch InRelease [15.0 kB]
Fetched 34.2 kB in 3s (8,771 B/s)
Reading package lists... Done
W: GPG error: http://download.mono-project.com/repo/debian stretch InRelease: Th e following signatures couldn't be verified because the public key is not availa ble: NO_PUBKEY A6A19B38D3D831EF
W: The repository 'http://download.mono-project.com/repo/debian stretch InReleas e' is not signed.
N: Data from such a repository can't be authenticated and is therefore potential ly dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration deta ils.
W: GPG error: https://apt.sonarr.tv master InRelease: The following signatures c ouldn't be verified because the public key is not available: NO_PUBKEY EBFF6B99D 9B78493
W: The repository 'https://apt.sonarr.tv master InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potential ly dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration deta ils.