Skip duplicate ID3v2 tags in MPEG files.

I created a new patch for #450 and got the output like this:
```
TagLib: ID3v2.4 no longer supports the frame type TDAT.  It will be discarded from the tag.
TagLib: MPEG::File::firstFrameOffset() - Duplicate ID3v2 tag found.
-- TAG (basic) --
title   - "No Ones Ugly After 2 AM"
artist  - "Left Wing Fascists"
album   - "All Fired Up"
year    - "1991"
comment - ""
track   - "7"
genre   - "Punk"
-- TAG (properties) --
ACOUSTID_ID                       - "45be336e-51da-4fb8-9d5d-11c52d7d80a1"
ALBUM                             - "All Fired Up"
ALBUMARTIST                       - "Left Wing Fascists"
ALBUMARTISTSORT                   - "Left Wing Fascists"
ARTIST                            - "Left Wing Fascists"
ARTISTSORT                        - "Left Wing Fascists"
DATE                              - "1991-01-01"
DISCNUMBER                        - "1/1"
GENRE                             - "Punk"
LABEL                             - "Skyrat Music"
MEDIA                             - "Digital Media"
MUSICBRAINZ ALBUM RELEASE COUNTRY - "US"
MUSICBRAINZ ALBUM STATUS          - "official"
MUSICBRAINZ ALBUM TYPE            - "album"
MUSICBRAINZ_ALBUMARTISTID         - "1fc55b7b-dec1-480f-8599-5c1ac8313c06"
MUSICBRAINZ_ALBUMID               - "fa0c535c-8bfa-4795-a26d-99cb6c8497e7"
MUSICBRAINZ_ARTISTID              - "1fc55b7b-dec1-480f-8599-5c1ac8313c06"
MUSICBRAINZ_RELEASEGROUPID        - "1b4fb8df-a3ca-4d00-b22b-8a5a2282b150"
MUSICBRAINZ_TRACKID               - "a00754f0-f4f4-4e7f-9cf7-72800fb9a474"
ORIGINALDATE                      - "1991"
SCRIPT                            - "Latn"
TITLE                             - "No Ones Ugly After 2 AM"
TRACKNUMBER                       - "7/11"
-- AUDIO --
bitrate     - 128
sample rate - 44100
channels    - 2
length      - 2:13
``` 
However, as I mentioned in #450, I'm not sure it's worth to merge. I'd like to wait the discussion there.
It would be better to avoid parsing the whole tag data, given that it needs just the header.