Dogmeat being able to take things from locked containers and duping fetched items was in fallout 3. Ten years and 3 games later it's unresolved. Maybe if they spent less time trying to put their immersion breaking purebred into the game and more ironing out the ancient game engine.

Maybe they just wanna put it in and don't care about it.

Besides, how a duped item can give unfair benefit you anyway?

You only can sell it for a petty cash or scrap it for little bundle of material at the end of the day.

man... if it can pin a deathclaw in place then it has every right to loot whatever it wants from any container