handling of completion table duplicates

[Ingo Lohmar]

Hi Everybody,

I recently noticed that ivy-mode (at its very core a completing-read
alternative, just like ido-completing-read is another one) does *not*
eliminate duplicates from the collection it is passed, eg, in form of a
plain list.

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

Both completing-read as well as ido-completing-read remove such
duplicates, but I could not find that behavior documented in the elisp
info manual or the docstrings.

Is removing duplicates from the collection *expected* from completion
functions, or is the caller of the function responsible for providing a
collection without duplicates?

If I did not just miss the documentation (in which case I apologize),
should this be documented, and where?

Ingo