Changes have been made in Git (FRAMEWORK_5_2):

commit 021a9060b563bf209bdeba1542f25c7d1db1ac0c
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Thu Oct 1 13:55:18 2015 -0400

     Add, and use, option to detect duplicate on import using any email field.

     Bug: 14119 Used when automatically adding recipients to default
     addressbook.

  imp/lib/Compose.php |   10 +++++++++-
  turba/lib/Api.php   |   25 +++++++++++++++++++++++--
  turba/lib/Turba.php |    1 +
  3 files changed, 33 insertions(+), 3 deletions(-)

http://github.com/horde/horde/commit/021a9060b563bf209bdeba1542f25c7d1db1ac0c

Changes have been made in Git (master):

commit f8caeaababc7dda92e4967f8711c37939397e03d
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Thu Oct 1 13:55:18 2015 -0400

     Add, and use, option to detect duplicate on import using any email field.

     Bug: 14119 Used when automatically adding recipients to default
     addressbook.

  imp/lib/Compose.php |   10 +++++++++-
  turba/lib/Api.php   |   24 ++++++++++++++++++++++--
  2 files changed, 31 insertions(+), 3 deletions(-)

http://github.com/horde/horde/commit/f8caeaababc7dda92e4967f8711c37939397e03d

It seems that the 'existence'  check on the basis of which a new 
contact can be automatically added to the address book (when the 
feature is enabled) is not performed on the pure email address (e.g. 
p.galeazzi@abanet.it)  but on the whole stuff (e.g. ABACO INFORMATICA 
- Paolo Galeazzi,   <p.galeazzi@abanet.it>).
This means that if I reply to a message where the sender has just 
modified a comma, ot if I type the recipient  address instead of 
picking it out of the drop down list, a new entry is added to the 
address book with the same email address: a duplicate.