Soap Sync service, Start time - End time issue

Please help me with this

how to pesist Duraton of Sync message processing?

This is a synchronous scenario, SOAP, sebservice, then I need statistics of processing time. Like this.

Then I can see Start Time and End Time different. And Duration is calculated right.

If the process sent more messages, then in pimon Start time and End time for old records become equals, and duration is 0.

The following imege show Same Message IDs with no duration, after sending more messages for the same interface

Thanks for your help