Thanks everybody for your advises. very helpful.
Though Saudi Airlines do not mention anything about this situation on their website and when i called their UK helpline, they were not very sure. I tried calling their Delhi(India) helpline. they said I should use Father/mother/spouse first name as passenger's last name in such situations where passenger only have single name in passport. A bit weird. Isn't?
They further added as the tickets have already been booked such a change would mean mean re-booking/re-issue of tickets.

I am really confused and disappointed as what i did (same name in first and last name) is a very common practice and followed every where. there was no error on my part, hence do not want to pay anything extra (re-booking/re-issue cost of tickets would be very high in my case)

Please advise if it is worth not going for a change now and look to get this sorted at airport (knowing this situation must be common and strictly speaking there was no error on my part). Further to add contacting Saudi UK office wasn't very helpful in first place, Saudi Airlines do not mention anything about this situation on their website and contact number of their Delhi office provided on website are incorrect.

Thanks
Amit