Does it actually work like that?
For example, Overwatch will happily give you duplicate results and just give you currency for it. Meanwhile, something like Fallout Shelter will avoid giving you duplicates in the same tier until you earn everything in that tier, then will starting giving duplicates that auto-convert to currency instead of a higher tier.
From what I have read it apparently does stop giving you duplicates once you have maxed them. The game being so new however I wouldn't necessarily trust that as fact as it has just been rumblings here and there on various websites, nothing official.
Like you said Overwatch and games like it turn your duplicates into currency. I have not heard anything about duplicates being turned to currency so I would wager it is because they don't give you them once they are maxed? (or again because the game is like a week old haha but I am sure there is someone who knows because they already have nearly everything.. o.O)
I personally have only bought the higher tier boxes myself anyway haha though I feel like I have convinced my own self to start just buying the cheaper ones. : P
Dug a bit deeper. PC Gamer says the following in one of their articles:
How to spend your currency
Stick with Basic packs until both your starting Common weapons and classes reach rank 10. You’ll see immediate benefits: each increasing class rank bestows a higher base skill point allotment to further augment your powers on a fresh character. Weapon stats will become better tuned for damage and weight efficiency. Most importantly, fully maxing your Common inventory removes their inclusion in future pack loot tables, increasing your chances of getting better gear from the more expensive packs.
After you’ve begun dipping into Silver matches, start buying Advanced packs to round out your Uncommon quality items and classes. Ideally, you want to mirror the same max-rank end-goal as you met for your Common stuff, but this is where the grind turns heavy; be patient and make sure to get your Apex dailies done for fat Credit bonuses. Once you hit rank 10 for your roster and armaments, you’re primed for the high-damage and high-reward big leagues.
Lastly, skip the Expert packs and go straight for the juicier Premium packs. For double the Credit price, you get a better chance at coveted Ultra-Rare unlocks alongside the two guaranteed and powerful Rare items. At this point, your class stable should be bristling with power and gear, and you shouldn’t have much trouble acing numerous Silver and Gold matches to dump Tempest-loads of Credits into your wallet. Continue using your excess Credits on Premium packs to hunt after specific weapons to your liking—some popular picks include the Mattock assault rifle, Equalizer pistol, Raptor sniper rifle, or Scattershot shotgun.
Again that isn't fact but it sounds good to me hah