As far as I know theres no plans to change that. Rerolling gives you guaranteed unowned content (if theres any available), so if you're unlucky with unboxing 3 times in a row theres a consolation prize essentially, but I feel like keeping the base chest contents uniformly random across all content (compared to just unowned) is a balancing tool so the 'rare' stuff stays rare.
Theres also the aspect that getting stuff you already own is useful for disenchanting, as you're unlikely to have any qualms disenchanting one of those (compared to disenchanting content you don't own, where you might have reservations about it in case you ever did decide to take up champ X).
I know they still want to do something about champ shards/blue essence to make those useful for players who already own all champs, but no word as to what/when AFAIK. I expect it'd still just be something with the shards/essence itself though, so nothing to save chests over.
It is somewhat frustrating I agree (I have 2 shen shards and own shen >.<), but I reckon that getting duplicates of stuff is just par for the course when it comes to random loot crate mechanics.