FLAC Xiphcomments addField doesn't allow duplicates for "standard" fields?

[Johan Helsingius]

Hi!

Trying to work with the tagpy python bindings, and trying
to add multiple "ARTIST" fields to FLAC files (compilations
also using "ALBUMARTIST").

When I try to use TagLib::Ogg::XiphComment::addField
(through the tagpy.FileRef.xiphComment.addField() binding),
and setting the third argument (the "replace" flag) to
false, everything works OK with user-specified tags,
but not with the basic fields such as ARTIST, ALBUM,
TITLE and DATE. If there is already a value set for
the tag, no replacement happens, but a new field isn't
added either.

Is this on purpose? On a quick scan of the source, I
couldn't find any code to prevent duplicates either
in taglib or in the python binding code.

	Julf