Steam now discounts bundles that contain games you already own
Steam game bundles are a fantastic way to expand your collection as they usually offer better value for money than buying each game individually.
The only problem with the bundles is that you'll usually have (at least) one of the games it contains in your Steam library already, which means if you purchase the bundle you'll pay again for a game you already own.
From now on, however, Steam will start automatically discounting the price of games already in your library from the total price of the bundle. Not all bundles seem to be affected by the change yet, so it may be gradually rolling out across the Steam Store.
Valve's official Developer Documentation confirms the change, as PCGN discovered:
"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."