I couldn't confirm this earlier due to Growl not working because of
the case-sensitive bug, but now I finally got Growl 1.3.3 running, so
I could check everything and confirm this.
I have two different user accounts - one for me, one for my wife. Most
of the time we are both logged in on our Mac.
Some time ago I've noticed that I'm getting notifications from
MenuWeather (
http://menuweather.com/mac) coming from my wife's user
account. It was easy to tell by the language of the notifications,
because I have my account set to English, while my wife has hers set
to Polish.
I first suspected it was the fault of MenuWeather (which I highly
recommend, by the way), but checking the history of Growl
notifications it turns out that I'm also getting duplicate hardware
notifications from HardwareGrowler running on the other account.
After further investigation I discovered that it's the first launched
instance of Growl that is receiving all the notifications. When a
different user logs in to another account, all the notifications are
passed to the first account.
You can easily check this by setting two different user accounts and
enabling Growl (with history) and HardwareGrowler on both. Log in to
the accounts and plug a USB memory stick. You will get all the
notifications on the account that you logged in to first.
Both of the accounts are non-admin, if that makes any difference.