Relationship types support relationship-ends that point to a first-class attribute in the entity-type of the end i.e. legacy attribute. For such relationship-ends, the associated entity will have the attribute included in both 'attributes' and 'relationshipAttributes'.
For relationships having legacy attributes at both ends, 2 edges will be created between the entities in this relationship. This was necessary earlier releases (prior to 1.0) due to use of isOwnedRef/inverseRef constraints, hence relationship feature was not available then. With introduction of relationships it shouldn't be necessary to use legacy attributes; the same can be modeled using COMPOSITION relationship.
To move away from earlier model (of using isOwnedRef/inverseRef), to use COMPOSITION relationship, updates to relationshipDef and entitiyDefs for both ends will be necessary. This is to track introducing a type-patch named REMOVE_LEGACY_ATTRIBUTES, along lines of ADD_ATTRIBUTE, UPDATE_ATTRIBUTE.