I think we should give the OP the benefit of the doubt. For all we know, a cowboy may have told him he needs an extra RCD, or he may just want to know what to ask for or expect when seeking quotes. Or he might be a letting agent wanting to know what to advise his clients. The possibilities are endless, we just don't know.
So with that, and my previous disclaimer in mind:
Alan:
A second 30mA RCD in series with the first will give no advantage as long as the first is working correctly. If it's not, it should be replaced. All the second would do is mean there were two places to check if one trips (undesirable).
Putting the lights in question on a separate circuit with its own separate RCD protection would give the advantage that if either RCD were to trip, lights and any other equipment on the other RCD would be unaffected. But it's not normal to put bathroom zone 1 lights on their own RCD like this. No-one can advise on the specifics without seeing it - there may be some reason why this would be appropriate in your particular installation. But a more typical arrangement for a domestic installation would be one lighting circuit per floor, often on different RCDs from one another. Once again, this is just typical, there may be reasons why your installation is different.
Finally, as others have pointed out, there are other requirements, such as (but not limited to) IP ratings, that need to be taken into account. I'm not going to address these here as they are beyond the scope of the thread (and I'm not going to teach you my whole trade for free!), but whoever is doing the work needs to understand them all and work to them.
Hope that's helpful.
Sam
