For some unknown reasons, my Windows XP SP2 is now showing everything in duplicate in my taskbar.

 

For example when I launched Firefox, I will see 2 Firefox "instance" in the taskbar instead of only 1.

 

To further clarify the problem, I attached here with an image of my desktop where the problems are shown with the red highlights.

 

 

What has caused this phenomenon which I call it a bug? Is there any solution to this?