Must close browser to properly log out

Although we have had an installation running here for a few years I've only recently noticed that if I try to log out of Xerte and then somebody else tries to log in using the same browser window, they see my content not their own, i.e. I haven't actually been logged out properly.

If I close the browser down after I log out the next person is able to open the browser and log in okay, though.

Is it normal to have to close the browser window to log out, or is something unusual going on here?

Thanks
Oli