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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.