Changes have been made in Git (FRAMEWORK_5_2):

commit 31d6aa47c86e87abba7e91b8369d38a2ec66bfc1
Author: Jan Schneider <jan@horde.org>
Date:   Mon, 10 Oct 2016 23:44:39 +0200

Reorganize vacation script.

Bug: 14486

Use elsifs and allof/anyof tests to better structure the test and to 
avoid double execution of vacation action.

  M lib/Script/Sieve/Action/Vacation.php

https://github.com/horde/ingo/commit/31d6aa47c86e87abba7e91b8369d38a2ec66bfc1

Changes have been made in Git (FRAMEWORK_5_2):

commit 6de87f162298a2e408d1b3008dc551cc16434798
Author: Jan Schneider <jan@horde.org>
Date:   Mon, 10 Oct 2016 23:45:36 +0200

[jan] Fix some edge cases with Sieve vacation rules with date limits 
(Bug #14486).

  M docs/CHANGES
  M package.xml

https://github.com/horde/ingo/commit/6de87f162298a2e408d1b3008dc551cc16434798

Changes have been made in Git (FRAMEWORK_5_2):

commit faeaad2098c3a168cb49e34adf89efc6a66d6ada
Author: Jan Schneider <jan@horde.org>
Date:   Mon Oct 10 23:45:36 2016 +0200

     [jan] Fix some edge cases with Sieve vacation rules with date 
limits (Bug #14486).

  ingo/docs/CHANGES | 2 ++
  ingo/package.xml  | 4 ++--
  2 files changed, 4 insertions(+), 2 deletions(-)

http://github.com/horde/horde/commit/faeaad2098c3a168cb49e34adf89efc6a66d6ada

Changes have been made in Git (FRAMEWORK_5_2):

commit 6f42d4dcbf21298631acae3ffe4ff63b3dae6dae
Author: Jan Schneider <jan@horde.org>
Date:   Mon Oct 10 23:41:57 2016 +0200

     Reorganize vacation script.

     Bug: 14486

     Use elsifs and allof/anyof tests to better structure the test and 
to avoid double execution of vacation action.

  ingo/lib/Script/Sieve/Action/Vacation.php | 127 
++++++++++++++++--------------
  1 file changed, 70 insertions(+), 57 deletions(-)

http://github.com/horde/horde/commit/6f42d4dcbf21298631acae3ffe4ff63b3dae6dae

Changes have been made in Git (master):

commit 1b5dd418cea0f289d156ba31c6abe069628d123a
Author: Jan Schneider <jan@horde.org>
Date:   Mon Oct 10 23:41:57 2016 +0200

     Reorganize vacation script.

     Bug: 14486

     Use elsifs and allof/anyof tests to better structure the test and 
to avoid double execution of vacation action.

  ingo/lib/Script/Sieve/Action/Vacation.php | 127 
++++++++++++++++--------------
  1 file changed, 70 insertions(+), 57 deletions(-)

http://github.com/horde/horde/commit/1b5dd418cea0f289d156ba31c6abe069628d123a

I think I found a bug, where the by ingo generated sieve script 
crashes with "error: duplicate vacation action not allowed"

I my case the user had the message for Sep 30 and Oct 1 activated, 
which generates following script part:
..................................................................
if header :regex "Received" "^.*(Sep) (\\(.*\\) )?.... (\\(.*\\) 
)?..:..:.. (\\(.*\\) )?((\\+|\\-)[[:digit:]]{4}|.{1,5})(\\(.*\\))?$" {
     if header :regex "Received" "^.*(30) (\\(.*\\) )?... (\\(.*\\) 
)?.... (\\(.*\\) )?..:..:.. (\\(.*\\) 
)?((\\+|\\-)[[:digit:]]{4}|.{1,5})( \\(.*\\))?$" {
     vacation :addresses ["..."] :subject "..." "...";
}
}
if header :regex "Received" "^.*(Oct) (\\(.*\\) )?.... (\\(.*\\) 
)?..:..:.. (\\(.*\\) )?((\\+|\\-)[[:digit:]]{4}|.{1,5})( \\(.*\\))?$" {
     if header :regex "Received" "^.*([0 ]1) (\\(.*\\) )?... (\\(.*\\) 
)?.... (\\(.*\\) )?..:..:.. (\\(.*\\) 
)?((\\+|\\-)[[:digit:]]{4}|.{1,5})( \\(.*\\))?$" {
     vacation :addresses ["..."] :subject "..." "...";
}
..................................................................

He received a message which contained following Received lines...

Received: ... Sat, 01 Oct 2016 03:26:19 +0200
Received: ... Fri, 30 Sep 2016 21:17:58 -0400

With such message all if clauses are true and the sieve script crashes 
at the second "vacation" action and the rest of the script isn't 
executed.

Solution:?
From my point of view the solution for the problem is to use elseif 
(elif) for checking the the second, third, ... month of the vacation 
message.