[Users] multiple 487 created

[unplug]

For the normal cancel situation.

UA1		OPENSER		UA2
--------INVITE----------------->
<-----------100-----------------
		    --------------------INVITE---------->
		    <---------------100-------------------
----------CANCEL---------->
		    --------------CANCEL------------->
		   <--------------200------------------
		  <------------487-------------------
		   --------------ACK----------------->
<---------487---------------
-------------ACK------------>

But in my case, I found that there are many 487 created from openser
to UA1.  The multiple 487 may cause a call drop in the following call.
 Why does openser create so many 487 for UA1?  How can I prevent it?

UA1		OPENSER		UA2
--------INVITE----------------->
<-----------100-----------------
		    --------------------INVITE---------->
		    <---------------100-------------------
----------CANCEL---------->
		    --------------CANCEL------------->
		   <--------------200------------------
		  <------------487-------------------
		   --------------ACK----------------->
<---------487---------------
-------------ACK------------>
<---------487-------------
-------------ACK------------>
<---------487---------------
-------------ACK------------>
<---------487-------------
-------------ACK------------>
<---------487-------------
-------------ACK------------>
<---------487-------------
-------------ACK------------>
<---------487-------------
-------------ACK------------>
...