When Mozilla is associated with both HTML pages and the http protocol, clicking on a URL in Outlook results two Mozilla windows with the same content. Deleting association with the http protocol (but leaving the association with HTML pages) solves the problem. Deleting association with HTML pages but leaving association with the http protocol does not solve the problem (still two windows appear). I use Outlook 2000 SR-1 9.0.0.4201 (german version). Of course it is possible that it is an error in Outlook if the problem does not exist with other third party mailers. The problem does not exist with the Mozilla Mail and News component.

sounds like another dup of bug 59078 Reporter: Please always include the Mozilla build ID in bug-reports.

Tried it, see this too, definitely a dup of bug 59078 *** This bug has been marked as a duplicate of 59078 ***