Specifying same named parameter multiple times is still allowed by bitcoin-cli.
The client implementation overwrites earlier option values with later ones
before sending to server. This is tested by interface_bitcoin_cli.py
Rationale for allowing client parameters to be specified multiple times in
bitcoin-cli is that this behavior has been supported for a long time, and that
when using the command line interactively, it can be convenient to override
earlier option values with new values without having to go back and remove the
old value.
But for the RPC server, there isn't really a good use-case for earlier values
to be discarded if multiple values are specified. JSON keys are generally
supposed to be unique and if they aren't it's probably an indication of some
problem generating the RPC request.
