Suggestion:
Allow unstackable exact duplicates to stack to 16 in slotted chests
Current behavior:
Unstackables will reserve a slot, but will only stack to 1
Reasoning:
This would facilitate simple and compact unstackable sorting setups as well as provide a great way to store duplicate unstackables, like enchanted books or tools.

It's already possible to sort unstackables using hopper filters, and slotted chests aren't meant for bulk storage like that.