docs: add clarity around bulk mode with duplicate IDs

docs/4.0. Behaviour.md

@@ -24,6 +24,8 @@ In environments where multiple clients may be operating against a single Connect
 
 Where a server implementation supports concurrent application of settings changes to underlying Senders and Receivers, it may choose to perform 'bulk' resource operations in a parallel fashion internally. This is an implementation decision and is not a requirement of this specification.
 
+If a 'bulk' request includes multiple sets of parameters for the same Sender or Receiver ID the behaviour is defined by the implementation. In order to maximise interoperability clients are encouraged not to include the same Sender or Receiver ID multiple times in the same 'bulk' request.
+
 # Scheduled Activations
 
 IS-05 provides a mechanism whereby the activation of staged transport parameters can be performed at a particular TAI time, or after a given interval.