diff --git a/doc/release-notes-26628.md b/doc/release-notes-26628.md
new file mode 100644
index 00000000000..48a07c1e818
--- /dev/null
+++ b/doc/release-notes-26628.md
@@ -0,0 +1,4 @@
+JSON-RPC
+---
+
+The JSON-RPC server now rejects requests where a parameter is specified multiple times with the same name, instead of silently overwriting earlier parameter values with later ones. (#26628)
diff --git a/src/rpc/client.cpp b/src/rpc/client.cpp
index b3434b80c7b..ea094976bf8 100644
--- a/src/rpc/client.cpp
+++ b/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;
diff --git a/src/rpc/server.cpp b/src/rpc/server.cpp
index 80a22ff8ca2..a026b7adfa2 100644
--- a/src/rpc/server.cpp
+++ b/src/rpc/server.cpp
@@ -399,7 +399,10 @@ static inline JSONRPCRequest transformNamedArguments(const JSONRPCRequest& in, c
     const std::vector<UniValue>& values = in.params.getValues();
     std::unordered_map<std::string, const UniValue*> argsIn;
     for (size_t i=0; i<keys.size(); ++i) {
-        argsIn[keys[i]] = &values[i];
+        auto [_, inserted] = argsIn.emplace(keys[i], &values[i]);
+        if (!inserted) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Parameter " + keys[i] + " specified multiple times");
+        }
     }
     // Process expected parameters. If any parameters were left unspecified in
     // the request before a parameter that was specified, null values need to be
diff --git a/src/test/rpc_tests.cpp b/src/test/rpc_tests.cpp
index 21ccbe96484..f9b8a473304 100644
--- a/src/test/rpc_tests.cpp
+++ b/src/test/rpc_tests.cpp
@@ -84,11 +84,15 @@ BOOST_FIXTURE_TEST_SUITE(rpc_tests, RPCTestingSetup)
 
 BOOST_AUTO_TEST_CASE(rpc_namedparams)
 {
-    const std::vector<std::string> arg_names{{"arg1", "arg2", "arg3", "arg4", "arg5"}};
+    const std::vector<std::string> arg_names{"arg1", "arg2", "arg3", "arg4", "arg5"};
 
     // 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 named argument specified multiple times raises an exception
+    BOOST_CHECK_EXCEPTION(TransformParams(JSON(R"({"arg2": 2, "arg2": 4})"), arg_names), UniValue,
+                          HasJSON(R"({"code":-8,"message":"Parameter arg2 specified multiple times"})"));
+
     // 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]");
 
@@ -100,7 +104,7 @@ BOOST_AUTO_TEST_CASE(rpc_namedparams)
     BOOST_CHECK_EXCEPTION(TransformParams(JSON(R"({"args": [1,2,3], "arg4": 4, "arg2": 2})"), arg_names), UniValue,
                           HasJSON(R"({"code":-8,"message":"Parameter arg2 specified twice both as positional and named argument"})"));
 
-    // Make sure extra positional arguments can be passed through to the method implemenation, as long as they don't overlap with named arguments.
+    // Make sure extra positional arguments can be passed through to the method implementation, as long as they don't overlap with named arguments.
     BOOST_CHECK_EQUAL(TransformParams(JSON(R"({"args": [1,2,3,4,5,6,7,8,9,10]})"), arg_names).write(), "[1,2,3,4,5,6,7,8,9,10]");
     BOOST_CHECK_EQUAL(TransformParams(JSON(R"([1,2,3,4,5,6,7,8,9,10])"), arg_names).write(), "[1,2,3,4,5,6,7,8,9,10]");
 }
diff --git a/test/functional/interface_bitcoin_cli.py b/test/functional/interface_bitcoin_cli.py
index b1369c2615c..90a543b51b9 100755
--- a/test/functional/interface_bitcoin_cli.py
+++ b/test/functional/interface_bitcoin_cli.py
@@ -90,6 +90,10 @@ class TestBitcoinCli(BitcoinTestFramework):
         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_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)
 
         self.log.info("Test -stdinrpcpass option")