doc: add note regarding template classes and duplicate TypeId allocation

doc/manual/source/new-models.rst

@@ -385,6 +385,29 @@ every class that defines a new GetTypeId method, and it does the actual
 registration of the class into the system.  The :ref:`Object-model` chapter
 discusses this in more detail.
 
+Note: Template classes should both export the instantiated template and call
+``NS_OBJECT_TEMPLATE_CLASS_DEFINE (TemplateClass, TemplateArgument);``
+to prevent the same template from being instantiated more than a single
+time in different modules. This prevents errors such as
+``Trying to allocate twice the same uid: TemplateClass<TemplateArgument>``.
+
+An example for the ``CounterCalculator<uint32_t>``::
+
+    //.h file
+    namespace ns3
+    {
+        extern template class CounterCalculator<uint32_t>;
+    }
+    //.cc file
+    #include <ns3/.h file>
+    namespace ns3
+    {
+        NS_OBJECT_TEMPLATE_CLASS_DEFINE (CounterCalculator, uint32_t);
+    }
+
+More details can be found in `issue #761 <https://gitlab.com/nsnam/ns-3-dev/-/issues/761>`_.
+
+
 Including External Files
 ++++++++++++++++++++++++