Unfortunately, I'm afraid that you cannot always assume this - it's not uncommon for airlines to have different baggage allowances for international flights (in your case the London to Mumbai leg) than for domestic flights (the Mumbai to Goa leg) and it doesn't always make a difference as to whether it is booked as a through fare with the same airline or not. They can and sometimes do set out different terms of carriage for international and domestic flights. I used to regularly fly to Cuba with Cubana - they had a 40kgs allowance for the international flights in and out of Cuba but for all domestic flights it is only 20kgs. The tickets would always be booked as 'through fares' as far as I remember and I'd rarely get charged for the baggage that was checked right through on the outbound journey but coming back I'd invariably get charged excess baggage for the connecting Santiago/Havana connecting flight. At only 1CUC per kilo (less than £1!) it never bothered me - especially as I would take the maximum allowance out due to gifts and taking things for friends but usually didn't exceed the 20kgs by too much on the return. I've had the same experience with BA in the past too - it's no longer an issue as they've reduced their luggage allowances for cattle class on international flights too now!

I've been trying to get into the Opodo site to see what it says and it appears to be down at the moment but on both Travel Republic when I've used them and on the BA and Cubana sites it has always been clear in the T&Cs that this could be the situation. But if someone just clicked and said they'd read them when they hadn't, they wouldn't necessarily have found this out this until the tickets arrived. If the Opodo site is set up in the same way then I don't think that there's much that you can do about it - it is never a good idea to just 'click' on that button and proceed with the booking without making sure that you do read the full T&Cs because a lot of this sort of thing is set out there rather than on the 'front' pages when making flight bookings. To stand any chance of pursuing this with Opodo, I think that you are going to have to prove that the two levels of baggage allowance are not mentioned anywhere in the T&Cs on the site and not just not on the actual booking pages.

SM