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.