completing-read displays duplicates in the *Completions* buffer

completing-read displays duplicates in the *Completions* buffer

[Kevin Rodgers]

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

RE: completing-read displays duplicates in the *Completions* buffer

[Drew Adams]

    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
    \---

I'm not convinced that this is a bug.

There is nothing in the doc of `completing-read' or `all-completions' that
suggests that these functions should remove duplicates. This has _always_
been the behavior of these functions, AFAIK. Changing this behavior could
break existing libraries (at least external libraries) that might depend on
it.

Niether of these functions removes duplicates, but you can make sure there
are no duplicates, if you like, by passing them an appropriate TABLE
argument. IOW, as the Doctor says, "If it hurts, don't do that!"