Hi K S
use this with count > 1 and not with subquery 
like Abhinab Mishra
my mind goes some where else.
So code its like that:
Select single blenr
MAX( gjahr )
INTO (lv_beln, lv_gjahr)
From besg
Where kostl = 'WRX'
GROUP BY belnr gjahr
HAVING COUNT( ktosl ) > 1.