Valve’s new pricing structure means you no longer need to pay for products you already own in Steam bundles.
The next time you purchase a Steam bundle you'll receive an automatic discount based upon which games are in your Steam library.
Bundle pricing will also factor in existing discounts during annual sales, so it will be easy to pick up a bargain.
Valve has released an official explanation on the new practice to developers:
"With Steam Bundles, if a customer already [owns] some items in the bundle, they will pay for and receive only the items not already in their account. This allows the best fans of your series or franchise to 'complete the set' and get a deal on the remaining items in the Bundle."
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The new pricing model is already in place with some bundles, and will likely roll out to more in the coming weeks.
For example, here is how much the Valve Complete Pack normally costs:
And here it is with the discount applied on my account:
It's unclear when this new system will be widely available, but the next big Steam sale is a safe bet.