No handling of valid repeated fields in extra metadata in ogg
When multiple fields with the same fieldname appear in vorbiscomments (in compliance with the specification), only the contents of one of these are displayed as media/codec metadata by VLC.
Vorbiscomments permit (and encourage)[1] the use of "repeated" metadata fields where appropriate - for example, a collaborative track by two artists should have two "artist=" metadata entries each with one of the artists' names, rather than one "artist=" tag with an undefined combination of the two artists.
(example: "artist=Joe Schmoe" and "artist=Richard Cephalus" rather than a single "artist=Richard Cephalus and Joe Schmoe")
This extends to "metadata_block_picture", which allows for multiple instances of embedded images ("album art"), using the same categorization as the id3v2.3 "APIC" frames do.
An example of an .opus file with two embedded images ("front cover" and "Publisher / Studio logotype" [pictype 20]) can be found here: http://opuscast.com/audio/48-06-28_FrankChandlerReturns-14.opus
I have no examples available of mp3 files with multiple embedded images currently, but I would assume the problem would extend to them as well, as multiple APIC frames[2] may be legally embedded in an mp3 file in the same way (and with the same minor restrictions e.g. no more than one of picture type "1")
(Suggestion: possibly easy(?) short-term fix for text fields would be to just concatenate contents of same field name for display in the existing VLC interface. This won't completely solve e.g. sorting by "artist" in VLC for Android though.)
(Other suggestion: Define a heirarchy of "picture types" as per id3v2.3 APIC/metadata_block_picture types specifications which consistently determine which picture is displayed in the one "album art" part of the UI, i.e. "if there's a "front cover" [picture type 3] VLC displays that, if not it looks for "other" [picture type 0] and displays that if it exists [and so on]". In other words if a second embedded image is found during parsing, if it is higher on the list than the first one it replaces it, otherwise it is dropped. This doesn't solve the issue of validly including multiple embedded images of the same type, but these are likely quite rare for now.)
[1] "Ogg Vorbis Documentation" http://xiph.org/vorbis/doc/v-comment.html (retrieved 2013-06-08 - see the "Implications" section beneath the "Field names")
[2] http://id3.org/id3v2.3.0#Attached_picture (retrieved 2013-06-08)