Function: list-colors-duplicates

Return a list of colors with grouped duplicate colors.
If a color has no duplicates, then the element of the returned list
has the form '(COLOR-NAME). The element of the returned list with
duplicate colors has the form '(COLOR-NAME DUPLICATE-COLOR-NAME ...).
This function uses the predicate `facemenu-color-equal' to compare
color names. If the optional argument LIST is non-nil, it should
be a list of colors to display. Otherwise, this function uses
a list of colors that the current display can handle.