Invoking "lightdm --test-mode" twice causes upstart to fill disk space

How to reproduce :
1) Open graphical user session
2) invoke "lightdm --test-mode" in a terminal
3) while it is running, invoke "lightdm --test-mode" in another terminal

What happens :
1) the second lightdm fails to start
2) the process "init --user" fills the file ~/.cache/gdm/session.log with the lines "A handler is already registered for /com/ubuntu/Upstart" until all disk space is exhausted.
Removing the file doesn't solve the problem, I have to reboot and remove it.

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: upstart 1.10-0ubuntu7
ProcVersionSignature: Ubuntu 3.8.0-33.48-generic 3.8.13.11
Uname: Linux 3.8.0-33-generic x86_64
ApportVersion: 2.12.5-0ubuntu2.2
Architecture: amd64
Date: Fri Jan 3 20:02:40 2014
MarkForUpload: True
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: upstart
UpgradeStatus: Upgraded to saucy on 2013-11-10 (53 days ago)
UpstartBugCategory: Session
UpstartRunningSessionCount: 1
UpstartRunningSessionVersion: init (upstart 1.10)
UpstartRunningSystemVersion: init (upstart 1.10)