diff --git a/doc/release-notes-26628.md b/doc/release-notes-26628.md
new file mode 100644
index 0000000000..48a07c1e81
--- /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/server.cpp b/src/rpc/server.cpp
index f57133f75b..232e2119f1 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 23e666bc45..a4d5a4b12c 100644
--- a/src/test/rpc_tests.cpp
+++ b/src/test/rpc_tests.cpp
@@ -89,8 +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 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]");