Prevent duplicate record labels in tags.

Picard receives Labels/Catalog Number pairs and separates these into
different tags. Where a two catalog numbers are from the same Label, the
label gets duplicated in the label tag, and this fix prevents that.
Prevent duplicate catalog numbers in tags

There are examples in MB database of two label/catalog numbers where
labels are different but catalog numbers are the same. Since these are
is stored in separate tags, this fix avoid duplicate catalog numbers.
Merge pull request #113 from Sophist-UK/sophist_prevent_duplicate_record_labels

Prevent duplicate record labels in tags.