I've rechecked my claim and I have some new information for you.
The option "Automatically put people I reply to in" no longer makes duplicate entries in the contact list. Here's my disjointed observations:
(1) I still have the Contact list where I observed the duplicate entries. There's three copies of the same email address.
(2) No new entries have been added to the Contact list for quite a while. I tested the action tonight and found that B&W was not adding new entries. I tried reconnecting to the Contact list through Options> Browse and tested again. B&W still failed to add email addresses to the Contact and I know for a fact I have never used these addresses before.
(3) I created a new Contact list, connected the B&W to it via the "Automatically put people I reply to in" Browse button. This time it worked!
On the face of it, the Contact list I used first would appear to be 'corrupted' so that B&W cannot write to it. There was no error message when I reconnected with B&W.
Here's the environemental factors:
A) I recently updated B&W to version 184.108.40.206 from the previous version. I think I started trying "Automatically put people I reply to in" after updating but I can't be certain.
B) The Contact list is a folder NOT in the default mailbox, but in a seperate PST file called, 'Common Resources.pst'.
C) In the same period that I was testing "Automatically put people I reply to in" I was also restructuring my PST files. Both the default mailbox and my 'Common Resources.pst' would have had their external filenames and internal folder names changed but I cannot recall the series of events.
D) Normally, my emails are sent immediately. If the link with the Maill (SMTP) server was broken so that messages couldn't be sent straightaway, there could have been three or more messages waiting to go. When does the "Automatically put people I reply to in" function operate? I wonder if it could be a timing issue related to when the messages actually get sent?