Library doesn't allow metadata with multiple values

[AlyoshaVasilieva]

A number of* EPUB's metadata attributes allow repeated elements, for example to define multiple authors:

<dc:creator opf:role="aut">Terry Pratchett</dc:creator>
<dc:creator opf:role="aut">Stephen Briggs</dc:creator>

and multiple genres:

<dc:subject>Fantasy</dc:subject>
<dc:subject>Humour</dc:subject>

(or multiple tags, common in web fiction and fanfiction)

However, this library only allows one instance of each. It's not clear to me what use there is for repeating some of the elements (different languages?) but I checked ebooks I own and found that the following are commonly repeated:

Element	Reason
creator	Multiple authors
subject	Multiple genres/tags
identifier	Different opf:schemes (uuid, ISBN, Amazon, ...)

It would be useful for this library to allow this.

*: See § 3.4.3 Metadata and particularly § 3.4.3.3 DCMES Optional Elements