You can't directly, for the entire column. However, indirectly, you can convert the first N characters and group by this instead, e.g.
SELECT CONVERT(NVARCHAR(100), MSGTEXT), COUNT(*) TotalCount
FROM MMOUTBOUND
GROUP BY CONVERT(NVARCHAR(100), MSGTEXT)
HAVING COUNT(*)>1;
As others have noted, note that you should convert your NTEXT
columns to NVARCHAR(MAX)