package ruby-json (not installed) failed to install/upgrade: Versuch, »/usr/lib/ruby/vendor_ruby/json.rb« zu überschreiben, welches auch in Paket ruby-json-pure 1.5.1-0ubuntu3 ist

No idea, what happened. I just tried to install the package as is by the help of the synaptics package management.

ProblemType: Package
DistroRelease: Ubuntu 12.04
Package: ruby-json (not installed)
ProcVersionSignature: Ubuntu 3.2.0-32.51-generic 3.2.30
Uname: Linux 3.2.0-32-generic x86_64
ApportVersion: 2.0.1-0ubuntu14
Architecture: amd64
Date: Tue Oct 9 09:07:51 2012
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-boulder-precise-amd64-20120529-0
ErrorMessage: Versuch, »/usr/lib/ruby/vendor_ruby/json.rb« zu überschreiben, welches auch in Paket ruby-json-pure 1.5.1-0ubuntu3 ist
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20120529-17:17
SourcePackage: ruby-json
Title: package ruby-json (not installed) failed to install/upgrade: Versuch, »/usr/lib/ruby/vendor_ruby/json.rb« zu überschreiben, welches auch in Paket ruby-json-pure 1.5.1-0ubuntu3 ist
UpgradeStatus: No upgrade log present (probably fresh install)