Make the JSON-RPC server reject requests with the same named parameter specified multiple times, instead of silently overwriting earlier parameter values with later ones.

Generally JSON keys are supposed to unique, and their order isn't supposed to be significant, so having the server silently discard duplicate keys is error-prone. Most likely if an RPC client is sending a request with duplicate keys it means something is wrong with the request and there should be an error.

After this change, named parameters are still allowed to specified multiple times on the bitcoin-cli command line, since bitcoin-cli automatically replaces earlier values with later values before sending the JSON-RPC request. This makes sense, since it's not unusual for the order of command line options to be significant or for later command line options to override earlier ones.

ninja all check-all