diff --git a/src/mcp/server/streamable_http.py b/src/mcp/server/streamable_http.py
index c1c8a0f61..63eff3750 100644
--- a/src/mcp/server/streamable_http.py
+++ b/src/mcp/server/streamable_http.py
@@ -288,6 +288,8 @@ def _create_error_response(
         status_code: HTTPStatus,
         error_code: int = INVALID_REQUEST,
         headers: dict[str, str] | None = None,
+        *,
+        request_id: RequestId | None = None,
     ) -> Response:
         """Create an error response with a simple string message."""
         response_headers = {"Content-Type": CONTENT_TYPE_JSON}
@@ -300,7 +302,7 @@ def _create_error_response(
         # Return a properly formatted JSON error response
         error_response = JSONRPCError(
             jsonrpc="2.0",
-            id=None,
+            id=request_id,
             error=ErrorData(code=error_code, message=error_message),
         )
 
@@ -436,7 +438,7 @@ async def _validate_accept_header(self, request: Request, scope: Scope, send: Se
             return False
         return True
 
-    async def _handle_post_request(self, scope: Scope, request: Request, receive: Receive, send: Send) -> None:
+    async def _handle_post_request(self, scope: Scope, request: Request, receive: Receive, send: Send) -> None:  # noqa: C901
         """Handle POST requests containing JSON-RPC messages."""
         writer = self._read_stream_writer
         if writer is None:  # pragma: no cover
@@ -523,6 +525,19 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
 
             # Extract the request ID outside the try block for proper scope
             request_id = str(message.id)
+
+            # Reject duplicate request IDs — a client that reuses an id while
+            # the original request is still in flight violates the JSON-RPC
+            # spec and would silently overwrite the prior stream slot.
+            if request_id in self._request_streams:
+                response = self._create_error_response(
+                    f"Conflict: Request ID {request_id!r} is already in flight on this session",
+                    HTTPStatus.CONFLICT,
+                    request_id=message.id,
+                )
+                await response(scope, receive, send)
+                return
+
             # Register this stream for the request ID
             self._request_streams[request_id] = anyio.create_memory_object_stream[EventMessage](0)
             request_stream_reader = self._request_streams[request_id][1]
diff --git a/tests/shared/test_streamable_http.py b/tests/shared/test_streamable_http.py
index 7ceac8e86..0a1efa8c7 100644
--- a/tests/shared/test_streamable_http.py
+++ b/tests/shared/test_streamable_http.py
@@ -2384,3 +2384,73 @@ async def asgi_receive() -> Message:
     assert body_chunks[-1] == {"type": "http.response.body", "body": b"", "more_body": False}
     assert "Error in standalone SSE writer" not in caplog.text
     assert "Error in standalone SSE response" not in caplog.text
+
+
+@pytest.mark.anyio
+async def test_duplicate_request_id_rejected_with_409() -> None:
+    """A POST with a request id already in _request_streams is rejected 409.
+
+    When a client reuses a JSON-RPC request id while the original request
+    is still in flight, the server must surface the violation instead of
+    silently overwriting the prior stream slot (gh-2655).
+    """
+    transport = StreamableHTTPServerTransport(
+        mcp_session_id="test-session",
+        security_settings=TransportSecuritySettings(enable_dns_rebinding_protection=False),
+    )
+    # Satisfy the read-stream guard so the POST handler proceeds.
+    read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
+    transport._read_stream_writer = read_stream_writer  # pyright: ignore[reportPrivateUsage]
+
+    # Seed an in-flight request stream so the duplicate check triggers.
+    duplicate_id = "dup-1"
+    seeded_send, seeded_receive = anyio.create_memory_object_stream[EventMessage](0)
+    transport._request_streams[duplicate_id] = (seeded_send, seeded_receive)  # pyright: ignore[reportPrivateUsage]
+
+    sent: list[Message] = []
+
+    async def asgi_send(message: Message) -> None:
+        sent.append(message)
+
+    async def asgi_receive() -> Message:
+        return {
+            "type": "http.request",
+            "body": json.dumps(
+                {
+                    "jsonrpc": "2.0",
+                    "method": "tools/call",
+                    "params": {"name": "test", "arguments": {}},
+                    "id": duplicate_id,
+                }
+            ).encode(),
+            "more_body": False,
+        }
+
+    scope: Scope = {
+        "type": "http",
+        "method": "POST",
+        "path": "/mcp",
+        "query_string": b"",
+        "headers": [
+            (b"accept", b"application/json, text/event-stream"),
+            (b"content-type", b"application/json"),
+            (b"mcp-session-id", b"test-session"),
+        ],
+    }
+
+    async with read_stream_writer, read_stream, seeded_send, seeded_receive:
+        with anyio.fail_after(5):
+            await transport.handle_request(scope, asgi_receive, asgi_send)
+
+    status = next(m["status"] for m in sent if m["type"] == "http.response.start")
+    assert status == 409
+
+    body = b"".join(m["body"] for m in sent if m["type"] == "http.response.body")
+    error_response = json.loads(body)
+    assert error_response["jsonrpc"] == "2.0"
+    assert error_response["id"] == duplicate_id
+    assert error_response["error"]["code"] == -32600
+    assert duplicate_id in error_response["error"]["message"]
+
+    # The original in-flight stream must not have been replaced.
+    assert duplicate_id in transport._request_streams  # pyright: ignore[reportPrivateUsage]