Right now, with two field info systems: The one for entity fields maintained by the EntityManager and the one for configurable fields. Ideally, we've only one and avoid duplicated field definitions in memory for the same thing (as we have it now).
As discussed in Prague the plan is to move all over to an entity field definition service (wherever that will resides). However, for that being possible we need to complete the following issues first:
#2047229: Change notice: Make use of classes for entity field and data definitions
#2114707: Allow per-bundle overrides of field definitions
Once we've that issue fixed, I think we've finally managed to unify both field APIs nicely.