bitcoin-cli: Make it an error to specify the "args" parameter two different ways

MarcoFalke reported the case of positional arguments silently overwriting the
named "args" parameter in bitcoin-cli
https://github.com/bitcoin/bitcoin/pull/19762#discussion_r1035761471 and this
behavior is confusing and was not intended when support for "args" parameters
was added to bitcoin-cli in #19762.

Instead of letting one "args" value overwrite the other in the client, just
pass the values to the server verbatim, and let the error be handled server
side.

src/rpc/client.cpp

@@ -289,6 +289,9 @@ UniValue RPCConvertNamedValues(const std::string &strMethod, const std::vector<s
         std::string name = s.substr(0, pos);
         std::string value = s.substr(pos+1);
 
+        // Intentionally overwrite earlier named values with later ones as a
+        // convenience for scripts and command line users that want to merge
+        // options.
         if (!rpcCvtTable.convert(strMethod, name)) {
             // insert string value directly
             params.pushKV(name, value);
@@ -299,7 +302,10 @@ UniValue RPCConvertNamedValues(const std::string &strMethod, const std::vector<s
     }
 
     if (!positional_args.empty()) {
-        params.pushKV("args", positional_args);
+        // Use __pushKV instead of pushKV to avoid overwriting an explicit
+        // "args" value with an implicit one. Let the RPC server handle the
+        // request as given.
+        params.__pushKV("args", positional_args);
     }
 
     return params;

test/functional/interface_bitcoin_cli.py

@@ -92,7 +92,7 @@ class TestBitcoinCli(BitcoinTestFramework):
 
         self.log.info("Test that later cli named arguments values silently overwrite earlier ones")
         assert_equal(self.nodes[0].cli("-named", "echo", "arg0=0", "arg1=1", "arg2=2", "arg1=3").send_cli(), ['0', '3', '2'])
-        assert_equal(self.nodes[0].cli("-named", "echo", "args=[0,1,2,3]", "4", "5", "6", ).send_cli(), ['4', '5', '6'])
+        assert_raises_rpc_error(-8, "Parameter args specified multiple times", self.nodes[0].cli("-named", "echo", "args=[0,1,2,3]", "4", "5", "6", ).send_cli)
 
         user, password = get_auth_cookie(self.nodes[0].datadir, self.chain)