Comment 5 for bug 388866

List of applets that open twice in Ubuntu 9.04 Desktop:

** PREFERENCES **
About Me
Appearance
Assistive Technologies
Database Access Properties
Default Printer
Display
Encryption and Keyrings
File Management
Keyboard
Keyboard Shortcuts
Main Menu
Mouse
Multimedia Systems Selector
Network Proxy
Onboard
Onboard Settings
Palm OS Devices
Popup Notifications
Power Management
Preferred Applications
Qt4 Settings
Removable Drives and Media
SCIM Input Method Setup
Screensaver
Search and Indexing
Sound
Startup Applications
Volume Control
Windows

** ADMINISTRATION **
Computer Janitor
Hardware Drivers
Language Support
Log File Viewer
Login Window
Network Tools
Printing
Services
Software Sources
System Testing (causes a popup error which requires mouse click to cancel second instantiation. This should not even occur. It should just not open in the first place)
Time and Date
USB Startup Disk Creator
Users and Groups

So as you can see, only half a dozen applets actually stopped itself from opening twice, and some of these better than others. Some (like Network) were flawless. Some caused a second 'Starting [application]' task bar window to appear and hang, before reverting to the primary window. Either way it is very inconsistent across the operating system.