don't think the wiring itself has to be RCD protected, portable appliances do, not sure about non-portable ones (but its a good idea to protect them)

putting two rcds in sequence is pretty pointless unless one is a time delay type.

putting outside sockets on the main house RCD is often done but a bad idea imo.

for a socket on the outside wall of the house my preffered way would be a RCD FCU indoors (fed from a non-rcd breaker in the CU), this will provide seperate RCD protection (less chance of accidently tripping all your sockets that way) and provide a conviniant place to isolate the socket to prevent electricity theft.

the time when outdoor rcd sockets are a good idea IMO is if the socket is a long distance from the nearest building, in a commercial environment RCD sockets also eliminate the issue of needing to get access to switchgear to reset RCDs.