Changes have been made in Git (FRAMEWORK_5_2):

commit 284d73a27a26e9edb31fe14888a9c233d0efcc04
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Mar 18 00:07:10 2015 -0400

     Start refactoring handling of exceptions/recurrence during save.

     This commit extracts commons iCal related code and wraps dav 
specific logic.
     (Hopefully) fixes Bug: 13898 and Bug: 13815.

     Still need to refactor other places we deal with exceptions.

     Conflicts:
             kronolith/package.xml

  kronolith/lib/Application.php |   60 
+++--------------------------------------
  kronolith/lib/Event.php       |   14 ---------
  kronolith/package.xml         |   10 ++++++-
  3 files changed, 13 insertions(+), 71 deletions(-)

http://github.com/horde/horde/commit/284d73a27a26e9edb31fe14888a9c233d0efcc04

Does this latest commit fix?

Changes have been made in Git (master):

commit dbc46bc90b2c4cd4f1a1efc3a65756fe3012f7d3
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Mar 18 00:07:10 2015 -0400

     Start refactoring handling of exceptions/recurrence during save.

     This commit extracts commons iCal related code and wraps dav 
specific logic.
     (Hopefully) fixes Bug: 13898 and Bug: 13815.

     Still need to refactor other places we deal with exceptions.

  kronolith/lib/Application.php |   60 
+++--------------------------------------
  kronolith/lib/Event.php       |   14 ---------
  kronolith/package.xml         |   14 ++++++++-
  3 files changed, 16 insertions(+), 72 deletions(-)

http://github.com/horde/horde/commit/dbc46bc90b2c4cd4f1a1efc3a65756fe3012f7d3

Cannot reproduce with current code. I've tried dozens of ways of 
moving an exception around to different dates/times and have not 
produced any duplicate entries.

I was able to reproduce some of the behavior of Bug: 13898 though 
(moving an instance of a count-limited series past the end of the 
existing series causes the exception to disappear).

Probably an issue with detecting the exception as an exception when 
it's edited a second time. Will look into.

Hi:

I have noticed an error using Thunderbird (Lightning) with events that 
are an exception of a recurrence event, where kronolith generates 
duplicate instances. Steps to reproduce the error:

1.- Connect your calendar with Thunderbird using CalDAV or WebDAV
2.- Create an event with 3 times repeating (in server or client)
3.- Move the second instance to another  hour or day to make an 
exception (in server or client)
4.-  In thunderbird , move the exception to another day and refresh 
kronolith. You we'll see a duplicated entry in kronolith, but only one 
instance in Thunderbird

Thanks