Single task to be done by multiple users

Only problem is it isn't completed. For example, I can right-mouse bottom click on the task, in Outlook's Task window, then select "Mark Complete". Outlook will close the window, but then immediately re-open it, telling me it needs attention; that it needs to be marked completed. This is the wrong behavior. When one marks a task completed, especially a task that isn't recurring, Outlook should save it as completed and never again pop up that task. But Outlook, for me, no longer behaves this way. Instead it closes it, then opens it, then I mark it completed, then it closes it, then it opens it, etc. I could, if I had the patience, do this 500 times and Outlook will never stop. It will never mark it completed and stop showing me the task again and again. The only way I can eventually stop Outlook is by deleting the task. This is NOT what I want to do!! I have no history of what I've done, if Outlook forces me to only delete tasks.