SELECT DISTINCT does not work. Why?

Question

Dear All,
I wrote the Query below in SAP B1 8.8 and it works fine.
But, for some strange reason, certain records are duplicated.
Could anybody explain why?
I tried to eliminate the duplicates by changing the 1st line of the Query from SELECT to SELECT DISTINCT
Error Message:
The ntext data type cannot be selected as DISTINCT because it is not comparable.
Could you help amend the Query?
Thanks
Leon Lai
SELECT
T0.[UpdateDate] AS 'Update Dt',
T0.[TaxDate] AS 'Doc Dt',
CASE T5.[TransType]
     WHEN '18' THEN 'PU ' + CONVERT(VARCHAR(6), T0.[DocNum])
     WHEN '19' THEN 'PC ' + CONVERT(VARCHAR(6), T0.[DocNum])
END 'SAP Ref.',
T1.[ImportLog]     AS 'Ship #',
T0.[CardCode] + '' AS 'Supplier #',
T0.[CardName]      AS 'Supplier Name',
T0.[DocTotal]      AS 'Rs',
T1.[BlockNum]      AS 'Reqn #',
T0.[DocNum]        AS 'Doc No',
T0.[U_SupInv]      AS 'Link'
FROM klship.[dbo].[OPCH] T0
INNER JOIN klship.[dbo].[PCH1] T1 ON T0.[DocEntry] = T1.[DocEntry]
INNER JOIN klship.[dbo].[OJDT] T5 ON T0.[TransID] = T5.[TransID]
WHERE
(T0.[UpdateDate] >= '[%2]' AND
T0.[UpdateDate]  <= '[%3]' AND
T0.[U_SupInv] IS NULL)
OR
(T0.[UpdateDate] >= '[%4]' AND
T0.[UpdateDate]  <= '[%5]' AND
T0.[U_SupInv] IS NOT NULL)
FOR BROWSE
Note: -  U_SupInv is a UDF used to attach a scanned pdf file to the Supplier's Invoice.

Answer 1

Dear István Korös,
Thanks a lot for your answer.
Your suggestion works!
I am closing this thread.
However, I have never met the problem of duplicate records before
with such straightforward Queries.
If you could explain to me why such duplicates arose, it would be very
satisfying.
Best Regards
Leon Lai