diff --git a/Assets/Mirror/Runtime/NetworkManager.cs b/Assets/Mirror/Runtime/NetworkManager.cs
index bcda61a9d..56e1815b4 100644
--- a/Assets/Mirror/Runtime/NetworkManager.cs
+++ b/Assets/Mirror/Runtime/NetworkManager.cs
@@ -141,6 +141,20 @@ public class NetworkManager : MonoBehaviour
         // virtual so that inheriting classes' OnValidate() can call base.OnValidate() too
         public virtual void OnValidate()
         {
+            // make sure someone doesn't accidentally add another NetworkManager
+            // need transform.root because when adding to a child, the parent's
+            // OnValidate isn't called.
+            foreach (NetworkManager manager in transform.root.GetComponentsInChildren<NetworkManager>())
+            {
+                if (manager != this)
+                {
+                    Debug.LogError($"{name} detected another component of type {typeof(NetworkManager)} in its hierarchy on {manager.name}. There can only be one, please remove one of them.");
+                    // return early so that transport component isn't auto-added
+                    // to the duplicate NetworkManager.
+                    return;
+                }
+            }
+
             // add transport if there is none yet. makes upgrading easier.
             if (transport == null)
             {