Distinct Clause not working in SQL 2005

I have so many records having duplicate taskid assigned to multiple person, but i want to show distinct records means only one taskid in output in SQL

below is my query not working give me solution

SELECT DISTINCT taskid, taskname, person, userid, dept, date, status, monitor, comments, monitor_comments, respondtime, assignedby, reassigncomment, priority,date_complete, followup_date, re_status
FROM         task
WHERE     (status IS NULL)