diff --git a/docs/DESIGN_AND_IMPLEMENTATION.md b/docs/DESIGN_AND_IMPLEMENTATION.md
index 4bd4655..2ec4a67 100644
--- a/docs/DESIGN_AND_IMPLEMENTATION.md
+++ b/docs/DESIGN_AND_IMPLEMENTATION.md
@@ -161,7 +161,6 @@ Invalid values are ignored and the default `header_with_nodetime_fallback` is us
   `GreenwaveMonitor::has_header_from_type()`; unknown types fall back to no-header behavior.
 - `publishDiagnostics()` marks status as `STALE` if no fresh `updateDiagnostics()` happened since the previous publish.
 - `setExpectedDt()` requires `expected_hz > 0`; zero disables useful timing checks.
-- Do not add externally monitored topics (those already using inline `GreenwaveDiagnostics`) via `manage_topic` or the terminal UI, as this will create a duplicate diagnostic subscription for the same topic.
 
 ## Where To Look In Code
 
diff --git a/greenwave_monitor/include/greenwave_monitor.hpp b/greenwave_monitor/include/greenwave_monitor.hpp
index 4302888..5d3389e 100644
--- a/greenwave_monitor/include/greenwave_monitor.hpp
+++ b/greenwave_monitor/include/greenwave_monitor.hpp
@@ -20,7 +20,9 @@
 #include <chrono>
 #include <map>
 #include <memory>
+#include <mutex>
 #include <optional>
+#include <set>
 #include <string>
 #include <thread>
 #include <vector>
@@ -48,6 +50,9 @@ class GreenwaveMonitor : public rclcpp::Node
     if (init_timer_) {
       init_timer_->cancel();
     }
+    // Reset diagnostics subscription before clearing internal state to prevent
+    // callbacks from firing after greenwave_diagnostics_ is destroyed
+    diagnostics_subscription_.reset();
     // Clear diagnostics before base Node destructor runs to avoid accessing invalid node state
     greenwave_diagnostics_.clear();
     subscriptions_.clear();
@@ -88,6 +93,8 @@ class GreenwaveMonitor : public rclcpp::Node
 
   void add_topics_from_parameters();
 
+  void diagnostics_callback(const diagnostic_msgs::msg::DiagnosticArray::SharedPtr msg);
+
   std::map<std::string,
     std::unique_ptr<greenwave_diagnostics::GreenwaveDiagnostics>> greenwave_diagnostics_;
   std::vector<std::shared_ptr<rclcpp::GenericSubscription>> subscriptions_;
@@ -98,4 +105,8 @@ class GreenwaveMonitor : public rclcpp::Node
   rclcpp::Service<greenwave_monitor_interfaces::srv::SetExpectedFrequency>::SharedPtr
     set_expected_frequency_service_;
   greenwave_diagnostics::TimeCheckPreset time_check_preset_;
+  rclcpp::Subscription<diagnostic_msgs::msg::DiagnosticArray>::SharedPtr
+    diagnostics_subscription_;
+  std::set<std::string> externally_diagnosed_topics_;
+  std::mutex externally_diagnosed_topics_mutex_;
 };
diff --git a/greenwave_monitor/src/greenwave_monitor.cpp b/greenwave_monitor/src/greenwave_monitor.cpp
index 48b26ef..28941f0 100644
--- a/greenwave_monitor/src/greenwave_monitor.cpp
+++ b/greenwave_monitor/src/greenwave_monitor.cpp
@@ -99,6 +99,14 @@ GreenwaveMonitor::GreenwaveMonitor(const rclcpp::NodeOptions & options)
   timer_ = this->create_wall_timer(
     1s, std::bind(&GreenwaveMonitor::timer_callback, this));
 
+  // Subscribe to /diagnostics early so we can detect external publishers before
+  // deferred_init() adds topics. This gives us the best chance of catching
+  // externally-published diagnostics before add_topic() is called.
+  diagnostics_subscription_ =
+    this->create_subscription<diagnostic_msgs::msg::DiagnosticArray>(
+    "/diagnostics", 10,
+    std::bind(&GreenwaveMonitor::diagnostics_callback, this, std::placeholders::_1));
+
   // Defer topic discovery to allow the ROS graph to settle before querying other nodes
   init_timer_ = this->create_wall_timer(
     100ms, [this]() {
@@ -170,6 +178,19 @@ void GreenwaveMonitor::timer_callback()
   RCLCPP_INFO(this->get_logger(), "====================================================");
 }
 
+void GreenwaveMonitor::diagnostics_callback(
+  const diagnostic_msgs::msg::DiagnosticArray::SharedPtr msg)
+{
+  std::lock_guard<std::mutex> lock(externally_diagnosed_topics_mutex_);
+  for (const auto & status : msg->status) {
+    // Only track topic names that are not already monitored by us. This prevents
+    // our own published diagnostics from blocking re-adds after a remove_topic().
+    if (greenwave_diagnostics_.find(status.name) == greenwave_diagnostics_.end()) {
+      externally_diagnosed_topics_.insert(status.name);
+    }
+  }
+}
+
 void GreenwaveMonitor::handle_manage_topic(
   const std::shared_ptr<greenwave_monitor_interfaces::srv::ManageTopic::Request> request,
   std::shared_ptr<greenwave_monitor_interfaces::srv::ManageTopic::Response> response)
@@ -319,6 +340,20 @@ bool GreenwaveMonitor::has_header_from_type(const std::string & type_name)
 bool GreenwaveMonitor::add_topic(
   const std::string & topic, std::string & message, int max_retries, double retry_wait_s)
 {
+  // Check if an external node is already publishing diagnostics for this topic.
+  // Adding a duplicate would create redundant and potentially conflicting diagnostics.
+  {
+    std::lock_guard<std::mutex> lock(externally_diagnosed_topics_mutex_);
+    if (externally_diagnosed_topics_.count(topic) > 0) {
+      message = "Topic is externally monitored";
+      RCLCPP_WARN(
+        this->get_logger(),
+        "Refusing to add topic '%s': topic is externally monitored",
+        topic.c_str());
+      return false;
+    }
+  }
+
   // Check if topic already exists
   if (greenwave_diagnostics_.find(topic) != greenwave_diagnostics_.end()) {
     message = "Topic already being monitored";
@@ -361,6 +396,18 @@ bool GreenwaveMonitor::add_topic(
 
 bool GreenwaveMonitor::remove_topic(const std::string & topic, std::string & message)
 {
+  {
+    std::lock_guard<std::mutex> lock(externally_diagnosed_topics_mutex_);
+    if (externally_diagnosed_topics_.count(topic) > 0) {
+      message = "Topic is externally monitored";
+      RCLCPP_WARN(
+        this->get_logger(),
+        "Refusing to remove topic '%s': topic is externally monitored",
+        topic.c_str());
+      return false;
+    }
+  }
+
   auto diag_it = greenwave_diagnostics_.find(topic);
   if (diag_it == greenwave_diagnostics_.end()) {
     message = "Topic not found";
diff --git a/greenwave_monitor/test/test_greenwave_monitor.py b/greenwave_monitor/test/test_greenwave_monitor.py
index 4fd10c4..2c8c8dd 100644
--- a/greenwave_monitor/test/test_greenwave_monitor.py
+++ b/greenwave_monitor/test/test_greenwave_monitor.py
@@ -23,6 +23,8 @@
 import time
 import unittest
 
+from diagnostic_msgs.msg import DiagnosticArray, DiagnosticStatus
+
 from greenwave_monitor.test_utils import (
     call_manage_topic_service,
     collect_diagnostics_for_topic,
@@ -415,6 +417,78 @@ def test_yaml_parameter_loading(self, expected_frequency, message_type, toleranc
             f'Topic {NONEXISTENT_TOPIC} should not be monitored'
         )
 
+    def test_reject_add_externally_diagnosed_topic(
+            self, expected_frequency, message_type, tolerance_hz):
+        """Test that add_topic() fails when the topic already has external diagnostics."""
+        if (message_type, expected_frequency, tolerance_hz) != MANAGE_TOPIC_TEST_CONFIG:
+            self.skipTest('Only running external diagnostics test once')
+
+        service_client = self.check_node_launches_successfully()
+        external_topic = '/external_diag_topic'
+        diag_pub = self.test_node.create_publisher(DiagnosticArray, '/diagnostics', 10)
+        try:
+            end_time = time.time() + 3.0
+            while time.time() < end_time:
+                msg = DiagnosticArray()
+                status = DiagnosticStatus()
+                status.name = external_topic
+                status.level = DiagnosticStatus.OK
+                status.message = 'External publisher'
+                msg.status = [status]
+                diag_pub.publish(msg)
+                rclpy.spin_once(self.test_node, timeout_sec=0.1)
+
+            # Greenwave Monitor should refuse to add the topic because an external
+            # node is already publishing diagnostics for it.
+            response = self.call_manage_topic(
+                add=True, topic=external_topic, service_client=service_client)
+            self.assertFalse(
+                response.success,
+                'add_topic() should fail when external diagnostics already exist for the topic'
+            )
+            self.assertIn(
+                'externally monitored', response.message.lower(),
+                'Error message should mention topic is externally monitored'
+            )
+        finally:
+            self.test_node.destroy_publisher(diag_pub)
+
+    def test_reject_remove_externally_diagnosed_topic(
+            self, expected_frequency, message_type, tolerance_hz):
+        """Test that remove_topic() fails when the topic is externally monitored."""
+        if (message_type, expected_frequency, tolerance_hz) != MANAGE_TOPIC_TEST_CONFIG:
+            self.skipTest('Only running external diagnostics removal test once')
+
+        service_client = self.check_node_launches_successfully()
+
+        external_topic = '/external_remove_topic'
+
+        diag_pub = self.test_node.create_publisher(DiagnosticArray, '/diagnostics', 10)
+        try:
+            end_time = time.time() + 3.0
+            while time.time() < end_time:
+                msg = DiagnosticArray()
+                status = DiagnosticStatus()
+                status.name = external_topic
+                status.level = DiagnosticStatus.OK
+                status.message = 'External publisher'
+                msg.status = [status]
+                diag_pub.publish(msg)
+                rclpy.spin_once(self.test_node, timeout_sec=0.1)
+
+            response = self.call_manage_topic(
+                add=False, topic=external_topic, service_client=service_client)
+            self.assertFalse(
+                response.success,
+                'remove_topic() should fail when topic is externally monitored'
+            )
+            self.assertIn(
+                'externally monitored', response.message.lower(),
+                'Error message should mention topic is externally monitored'
+            )
+        finally:
+            self.test_node.destroy_publisher(diag_pub)
+
 
 if __name__ == '__main__':
     unittest.main()