package readline-common 7.0-3 failed to install/upgrade: package readline-common is already installed and configured

My upgrade from Xubuntu Studio Artful Aardvark to Bionic Beaver failed early in the configuration stage, with no explanation. I repaired with ""apt --fix-broken installation" and seem to have a working system now. A lot of automatic problem reports were generated, and I have no idea what caused them.

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: readline-common 7.0-3
ProcVersionSignature: Ubuntu 4.15.0-20.21-lowlatency 4.15.17
Uname: Linux 4.15.0-20-lowlatency x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7
AptdaemonVersion: 1.1.1+bzr982-0ubuntu19
Architecture: amd64
Date: Thu May 10 21:40:13 2018
DuplicateSignature:
 package:readline-common:7.0-3
 Unpacking fonts-smc-uroob (2.0-1) ...
 dpkg: error processing package libmpfr6 (--configure):
  no package named 'libmpfr6' is installed, cannot configure
ErrorMessage: package readline-common is already installed and configured
InstallationDate: Installed on 2017-09-04 (248 days ago)
InstallationMedia: Xubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
PackageArchitecture: all
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: dpkg
Title: package readline-common 7.0-3 failed to install/upgrade: package readline-common is already installed and configured
UpgradeStatus: Upgraded to bionic on 2018-05-11 (0 days ago)