As already mentioned in this other thread, the detection of the unique visitors is not 100% reliable. I have just seen the following in my live view ( I was the visitor in this case) :
http://img268.imageshack.us/img268/2144/screenshottkf.png
Clearly, it should have been one unique visit.
I could be that it happens more often when the user works simultaneously in different browser windows.
Do you experience the same thing ? Is it a known bug ?
Thanks !