package libgnutls30:amd64 3.5.18-1ubuntu1 failed to install/upgrade: package libgnutls30:amd64 is already installed and configured

I started the upgrade process to Ubuntu 18.04, but the upgrade aborted midway during the installations because a number of packages could not be installed/upgraded for unknown reasons. There were several pop-up dialog boxes, one for each package that failed to install. The upgrade itself aborted, and when I restarted the system, my Ubuntu OS was broken and I could not login. I fired up the shell (Ctrl+Alt+F2) and ran "sudo apt --fix-broken install", followed by "sudo apt full-upgrade" and "sudo apt autoremove". Since all the packages were already downloaded, the three commands proceeded smoothly. Then when I rebooted, I was able to login to Ubuntu 18.04, so it seems like the upgrade was successful. But now I've gotten numerous crash dialog boxes ("Ubuntu has experienced an internal error"), one for each package that had failed to install during the upgrade process. One of them was "gnutls28". I am afraid that the system might still be broken here or there.

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: libgnutls30:amd64 3.5.18-1ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-22.24-generic 4.15.17
Uname: Linux 4.15.0-22-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
AptdaemonVersion: 1.1.1+bzr982-0ubuntu19
Architecture: amd64
Date: Sat Jun 9 10:57:06 2018
DuplicateSignature:
 package:libgnutls30:amd64:3.5.18-1ubuntu1
 Unpacking apt (1.6.1) over (1.5.1) ...
 dpkg: error processing package libtasn1-6:amd64 (--configure):
  package libtasn1-6:amd64 is already installed and configured
ErrorMessage: package libgnutls30:amd64 is already installed and configured
InstallationDate: Installed on 2016-06-19 (720 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
Python3Details: /usr/bin/python3.6, Python 3.6.5, python3-minimal, 3.6.5-3
PythonDetails: /usr/bin/python2.7, Python 2.7.15rc1, python-minimal, 2.7.15~rc1-1
RelatedPackageVersions:
 dpkg 1.19.0.5ubuntu2
 apt 1.6.1
SourcePackage: gnutls28
Title: package libgnutls30:amd64 3.5.18-1ubuntu1 failed to install/upgrade: package libgnutls30:amd64 is already installed and configured
UpgradeStatus: Upgraded to bionic on 2018-06-09 (0 days ago)