UPgrading from 18.04 to 20. upgrade fails saying ppa needs purging. Purge completed via command line or gui, but ppa seems to reappear, and the same error occurs with the upgrade. Impossible to upgrade

[Mredd]

UPgrading from 18.04 to 20. upgrade fails saying ppa needs purging. Purge completed via command line or gui, but ppa seems to reappear, and the same error occurs with the upgrade. Impossible to upgrade

"This was likely caused by:
* Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If
you want to investigate this yourself the log files in
'/var/log/dist-upgrade' will contain details about the upgrade.
Specifically, look at 'main.log' and 'apt.log'.
"

history | grep add-apt-repository

tells me:

  989 sudo add-apt-repository "deb http://realsense-hw-public.s3.amazonaws.com/Debian/apt-repo bionic main" -u

Have done:

sudo ppa-purge -s realsense-hw-public.s3.amazonaws.com ppa:Debian/apt-repo
sudo ppa-purge -s realsense-hw-public.s3.amazonaws.com ppa:Debian/apt-repo xenial
sudo ppa-purge -s realsense-hw-public.s3.amazonaws.com ppa:Debian/apt-repo bionic main

and a variety of others.

repos seem to return.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-1050.57-oem 4.15.18
Uname: Linux 4.15.0-1050-oem x86_64
NonfreeKernelModules: lkp_Ubuntu_4_15_0_1050_57_oem_59 nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Tue Oct 13 09:43:23 2020
DistributionChannelDescriptor:
 # This is the distribution channel descriptor for the OEM CDs
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-bionic-amd64-20180608-47+whitehaven-mlk+X67
InstallationDate: Installed on 2019-08-06 (433 days ago)
InstallationMedia: Ubuntu 18.04 "Bionic" - Build amd64 LIVE Binary 20180608-09:38
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-12 (0 days ago)
mtime.conffile..etc.update-manager.release-upgrades: 2020-10-13T08:18:58.305753