To synchronize MgdSchema?'s features a bit with JCR Node Type Notation, we should support multivalued properties.
The implementation most likely needs to have a multilang-like additional table where the different values are stored, and from where they are then queried with SQL JOINs.
Something like:
<type name="midgard_person">
...
<property name="email" type="string" multiple="true"/>
</type>
$person = new midgard_person();
$person->email[] = 'user@domain.com';
$person->email[] = 'firstname.lastname@domain.com';
$person->create();
$qb = new midgard_query_builder('midgard_person');
$qb->add_constraint('email', '=', 'user@domain.com');
$persons = $qb->execute();
echo $persons[0]->email[0]; // will output user@domain.com
