From the AMA - Alchemy building first with a target of summer. Hero academy second with a target of before Christmas. They wouldn’t commit to having a feature that would let you trade dups in, because they hadn’t finalized design. The advice in the AMA was not to save dups in anticipation of this feature (though you night have other reasons to).
While things might have changed, my guess is they haven’t put a ton more though in on the academy since the runway is so long and they have other features coming sooner.
As for me, i have some dups of “not so great” 5 stars that I’m keeping because I almost never feed a 5 star to another hero. So I have 3 Justices, and 3 Elena’s, 2 Elks, etc…you just never know what buffs will happen, or how skins may change the usefulness of a hero.
Uninteresting 4 star dups I feed off quickly and without thinking about it.