test: Add RPC tests for same named parameter specified more than once

Current behavior isn't ideal and will be changed in upcoming commits, but it's
useful to have test coverage regardless.

MarcoFalke reported the case of bitcoin-cli positional arguments overwriting
the named "args" parameter in
bitcoin/bitcoin#19762 (comment)

src/test/rpc_tests.cpp

@@ -89,6 +89,9 @@ BOOST_AUTO_TEST_CASE(rpc_namedparams)
     // Make sure named arguments are transformed into positional arguments in correct places separated by nulls
     BOOST_CHECK_EQUAL(TransformParams(JSON(R"({"arg2": 2, "arg4": 4})"), arg_names).write(), "[null,2,null,4]");
 
+    // Make sure later named argument value silently overwrites earlier values
+    BOOST_CHECK_EQUAL(TransformParams(JSON(R"({"arg2": 2, "arg2": 4})"), arg_names).write(), "[null,4]");
+
     // Make sure named and positional arguments can be combined.
     BOOST_CHECK_EQUAL(TransformParams(JSON(R"({"arg5": 5, "args": [1, 2], "arg4": 4})"), arg_names).write(), "[1,2,null,4,5]");
 

test/functional/interface_BGL_cli.py

@@ -99,6 +99,10 @@ def run_test(self):
         assert_raises_rpc_error(-8, "Parameter arg1 specified twice both as positional and named argument", self.nodes[0].cli.echo, 0, 1, arg1=1)
         assert_raises_rpc_error(-8, "Parameter arg1 specified twice both as positional and named argument", self.nodes[0].cli.echo, 0, None, 2, arg1=1)
 
+        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'])
+
         user, password = get_auth_cookie(self.nodes[0].datadir, self.chain)
 
         self.log.info("Test -stdinrpcpass option")