Ability to keep meta data if there are duplicate keys

Sample meta-data from an EPUB with this issue:
<dc:identifier opf:scheme="ISBN">9780316219259</dc:identifier>
<dc:identifier opf:scheme="MOBI-ASIN">B00BWQW73E</dc:identifier>
<dc:identifier id="uuid_id" opf:scheme="uuid">877a0722-d692-48f4-b491-40a0af0157e1</dc:identifier>
<dc:identifier opf:scheme="calibre">877a0722-d692-48f4-b491-40a0af0157e1</dc:identifier>

The current implementation of meta data will only return the last id (the calibre id) instead of all 4 entries.  I am appending the opf:scheme stringValue to make it a unique key.

Let me know if this requires any changes.  I can write a unit test if necessary and add this sample meta-data to the unit testing resources.