completing-read displays duplicates in the *Completions* buffer

[Kevin Rodgers <ihs_4664@yahoo.com>]

Stefan asked me to report this bug:

In GNU Emacs 21.4.1 (i386-pc-solaris2.8, X toolkit, Xaw3d scroll bars)
  of 2005-09-21 on daedal
configured using `configure  --prefix=/opt/csw'

Evaluate the following, then type `?' at the prompt:

(completing-read "Prompt: " '(("a") ("b") ("c") ("a") ("b") ("c")))

The *Completions* buffer contents are:

/---
| Click <mouse-2> on a completion to select it.
| In this buffer, type RET to select the completion near point.
|
| Possible completions are:
| a                                  a
| b                                  b
| c                                  c
\---

Even if the code is changed to use eq strings or eq sublists, the
duplicates are displayed:

(let ((a "a") (b "b") (c "c"))
   (completing-read "Prompt: "
                    (list (list a) (list b) (list c)
                          (list a) (list b) (list c))))

(let ((a (list "a")) (b (list "b")) (c (list "c")))
   (completing-read "Prompt: "
                    (list a b c a b c)))

-- 
Kevin Rodgers