A friend is planning to use a low cost airline for a travel from Tiruchirappalli (TRZ), India to Singapore with Air Asia, though his booking is combined there is a stopover for a short while at Kuala Lumpur. Considering the fact that our request is for a single journey and luggages are only handed to us at the destination, will the checked baggages be generally charged twice or would we pay only for one journey.
If the air travel is all on Air Asia and you bought this as a "Fly-Thru" service, then your bags will be checked through to Singapore, and you will only pay once, at check-in. Otherwise it will be considered two separate trips, you will pay twice, your bags will only be checked from TRZ to KUL, and you will have to re-check them in KUL.
See this Air Asia baggage page.