Distinct & Dynamic ORDER BY

Hello,

I have a stored procedure with dynamic ORDER BY. I would like to use the DISTINCT too. Is it somehow possible?
Thank you

Here is the stored procedure:

SELECT identifier_company + cast(identifier_number as nvarchar(3)) as identifier,
CASE WHEN canceled = 'True' THEN 'canceledPO' ELSE '' END AS style,
staff.staff_name,
purchase.purchase_id,
purchase.traveller_name,
nominal_department.department_name,
purchase.canceled,
purchase.travel_date,
convert(nvarchar(20), purchase.date_raised, 103) as dated,
supplier
FROM purchase INNER JOIN purchase_project ON purchase.purchase_id = purchase_project.purchase_id
INNER JOIN staff ON purchase.raised = staff.staff_id
INNER JOIN nominal_department ON purchase.department = nominal_department.nominal_dep_id
WHERE (raised in (SELECT staff_id FROM staff WHERE department like @FromDepartment) or purchase.raised = @raisedBy)
and purchase_project.project_number like '%' + @Query + '%'
ORDER BY
CASE @SortDir
WHEN 'ASC' THEN
CASE @OrderBy
WHEN 'staff_name' THEN cast(staff_name as nvarchar(100))
WHEN 'traveller_name' THEN cast(traveller_name as nvarchar(100))
WHEN 'department_name' THEN cast(department_name as nvarchar(100))
WHEN 'supplier' THEN cast(supplier as nvarchar(100))
WHEN 'canceled' THEN cast(canceled as nvarchar(10))
END
END
ASC,
CASE @SortDir
WHEN 'DESC' THEN
CASE @OrderBy
WHEN 'staff_name' THEN cast(staff_name as nvarchar(100))
WHEN 'traveller_name' THEN cast(traveller_name as nvarchar(100))
WHEN 'department_name' THEN cast(department_name as nvarchar(100))
WHEN 'supplier' THEN cast(supplier as nvarchar(100))
WHEN 'canceled' THEN cast(canceled as nvarchar(10))
END
END
DESC