Bug: Repeated candidate tags in org-set-tags-command interface [8.3.4 (8.3.4-31-gcb683e-elpa @ /home/jorge/.emacs.d/elpa/org-20160404/)]

[Jorge Peixoto de Morais Neto <jorge13515@gmail.com>]

I have set org-tag-persistent-alist.  In my .org files, many of these
tags do in fact appear.  But then when I try to change the tags of some
entry with org-set-tags-command, Org shows about double the expected
number of candidate tags, because those tags that occur both in org-
tag-persistent-alist and in the .org file appear twice, with different
access chars.  I expected no duplicates.  This bug does not occur when
I set org-tag-alist instead of org-tag-persistent-alist.

To ease debugging, I have created a new .emacs.d and removed all
unnecessary customizations (but still included a copy of my usual
~/.emacs.d/elpa).  I have also reduced the .org file to a minimum.  I
provide them as a compressed archive (.txz) here:
https://www.dropbox.com/s/wwo63h5e4f1k347/org-bug-repeated-candidate-ta
gs.txz?dl=1
I can also mail it on request (it is about 4MiB)

To trigger the bug, open GNU Emacs with my .emacs.d, visit
repeated-tags-bug-trigger.org and invoke org-set-tags-command.  The tag
"dummy" appears twice (once with access char "d" and another time with
access char "a").

I am Brazilian.  I apologize for possibly bad English and I welcome
corrections.

I use Ubuntu 15.10.

Emacs  : GNU Emacs 25.0.92.1 (x86_64-unknown-linux-gnu, GTK+ Version
3.18.6)
 of 2016-03-05
Package: Org-mode version 8.3.4 (8.3.4-31-gcb683e-elpa @
/home/jorge/.emacs.d/elpa/org-20160404/)

current state:
==============
(setq
 org-tab-first-hook '(org-hide-block-toggle-maybe org-babel-hide-
result-toggle-maybe org-babel-header-arg-expand)
 org-speed-command-hook '(org-speed-command-default-hook org-babel-
speed-command-hook)
 org-occur-hook '(org-first-headline-recenter)
 org-metaup-hook '(org-babel-load-in-session-maybe)
 org-confirm-shell-link-function 'yes-or-no-p
 org-after-todo-state-change-hook '(org-clock-out-if-current)
 org-src-mode-hook '(org-src-babel-configure-edit-buffer org-src-mode-
configure-edit-buffer)
 org-agenda-before-write-hook '(org-agenda-add-entry-text)
 org-babel-pre-tangle-hook '(save-buffer)
 org-mode-hook '(#[nil "\300\301\302\303\304$\207" [org-add-hook
change-major-mode-hook org-show-block-all append local] 5]
		 #[nil "\300\301\302\303\304$\207" [org-add-hook
change-major-mode-hook org-babel-show-result-all append local] 5]
		 org-babel-result-hide-spec org-babel-hide-all-hashes)
 org-archive-hook '(org-attach-archive-delete-maybe)
 org-ctrl-c-ctrl-c-hook '(org-babel-hash-at-point org-babel-execute-
safely-maybe)
 org-cycle-hook '(org-cycle-hide-archived-subtrees org-cycle-hide-
drawers org-cycle-show-empty-lines
		  org-optimize-window-after-visibility-change)
 org-confirm-elisp-link-function 'yes-or-no-p
 org-metadown-hook '(org-babel-pop-to-session-maybe)
 org-tag-persistent-alist '(("dummy" . 100))
 org-clock-out-hook '(org-clock-remove-empty-clock-drawer)
 )