[O] [BUG] Duplicate characters in help string when filtering by tag

[Matt Lundin]

I'm finding that when I call org-agenda-filter-by-tag, the string of
character hints is long, with lots of duplicates. E.g.,

Filter by tag [hpcwhpcwhpcwhpcwhpcwhpcwhpcwhpcwhpcwhpcw ]:tag-char, [TAB]:tag, 
[/]:off, [+/-]:filter/exclude, [q]:quit [3 times]

A git bisect reveals that this bug began with commit
2056b60aae528d0ba663d8b3804dc69e98993f1c. After a bit of investigating,
I've found the cause: org--tag-add-to-alist no longer removes duplicate
tags if they belong in a tag group. 

Let me provide a minimal example to illustrate. Take the following
setting:

(setq org-tag-alist
      '((:startgroup)
        ("home" . ?h)
        ("phone" . ?p)
        ("computer" . ?c)
        ("work" . ?w)
        (:endgroup)))

If one has 10 agenda files (none of which has a local #+TAGS
declaration), org-tag-alist-for-agenda will end up containing those tags
multiple times:

--8<---------------cut here---------------start------------->8---
org-tag-alist-for-agenda is a variable defined in ‘org.el’.
Its value is
(#1=(:startgroup)
    #2=("home" . 104)
    #3=("phone" . 112)
    #4=("computer" . 99)
    #5=("work" . 119)
    #6=(:endgroup)
    #1# #2# #3# #4# #5# #6# #1# #2# #3# #4# #5# #6# #1# #2# #3# #4# #5# #6# #1# 
#2# #3# #4# #5# #6# #1# #2# #3# #4# #5# #6# #1# #2# #3# #4# #5# #6# #1# #2# #3# 
#4# #5# #6# #1# #2# #3# #4# #5# #6# #1# #2# #3# #4# #5# #6#)
--8<---------------cut here---------------end--------------->8---

As a result, the help prompt for org-filter-tags-for-agenda looks like
this (notice the duplicate characters):

Filter by tag [hpcwhpcwhpcwhpcwhpcwhpcwhpcwhpcwhpcwhpcw ]:tag-char, [TAB]:tag, 
[/]:off, [+/-]:filter/exclude, [q]:quit [3 times]

Best,
Matt