Clicking link to Moodle in MS Excel results in "You are already logged in" message

If you click a hyperlink via Excel to a course in Moodle, if:

• you are NOT already logged in: the log in page appears, once you have logged then it takes you to the home page of Moodle. (It does NOT take you directly to the course).
• you ARE already logged in: the log in page appears with the following message "You are already logged in as #####, you need to log out before logging in as different user.", with two options:
  • Log out (if you click it, you log out)
  • Cancel (if you click it, the log in page appears)

This behaviour has been reproduced by Excel and Firefox (both 2.8.12 and 3.0.4). Libreoffice has not got this issue at least with Firefox.