Join the same table back to itself. Use an inner join so that rows that don't match are discarded. In the joined set, there will be rows that have a matching ARIDNR in another row in the table with a different LIEFNR. Allow those ARIDNR to appear in the final set.
SELECT * FROM YourTable WHERE ARIDNR IN (
SELECT a.ARIDNR FROM YourTable a
JOIN YourTable b on b.ARIDNR = a.ARIDNR AND b.LIEFNR <> a.LIEFNR
)