Irctc — TRAIN TICKET

I HAVE BOOKED TICKET FOR TIRUPAIT FROM ARAKKONAM ON 15/4/09 AT 5:38 P.M
THROUGH IRCTC, I GET CONFIRMATION FOR MY E-TICKET - SEAT NUMBER- D1-36, 37, 38
BUT WHEN I CHECK IN THE COMPARTMENT, I FOUND SOMEONE OCCUPIED
MY SEAT WITH THE SAME SEAT NO ON HIS TICKET, HOW THIS KIND OF
PROBLEM HAPPENS, I NEED PROPER REPLY FOR THIS PROBLEM FROM IRCTC.