What happens with cabin luggage allowance in indirect British Airways Flight?

Hi! I'm planning to travel with British Airways to Kolkata(India) because of its generous 23 kg cabin bag allowance .However since its an indirect flight ,I'll be transferred to an Air India domestic flight once I arrive in Delhi.My question is regarding my cabin bag allowance .The cabin bag allowance for Air India is very low (8kg) .What happens to the extra luggage weight? Will it be allowed? Many thanks for your help!

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