Titan has the notion of multi-properties which is not a part of the Blueprints API. A multi-property "allows a list of values on this property key for each vertex. This is useful when a property key is multi-valued, like "email" for example, since a user can have multiple email addresses".
If you use multi-properties then you can only set them via TitanVertex.addProperty
. You can't set them via any Blueprints methods.