bug#9722: list-colors-duplicates does not exclude enough colors on Windows

[Eli Zaretskii]

> From: Juanma Barranquero <lekktu@gmail.com>
> Date: Tue, 11 Oct 2011 00:49:43 +0200
> 
> On Windows, `list-colors-duplicates' matches the color name against
> the output of `w32-default-color-map', to avoid conflating colors
> which are RGB-equal, but semantically different, like SystemMenuText
> and SystemWindowText.
> 
> Unfortunately, that makes colors not in that list different even if
> they are not, in particular all the grayNN/greyNN pairs.

The duplicates annoyed me as well; thanks for taking care of it.

However, there's something I'm missing here: why doesn't
list-colors-duplicates recognize grayNN and greyNN as duplicates?
We don't have them in the list that w32-default-color-map returns.

As for duplicates such as "Dark Slate Gray" and "Dark Slate Grey",
which are not filtered out because they _are_ in
w32-default-color-map, would something break if we modify the list
returned by that function to not include any duplicates in the first
place?