It looks like it just double rewards, no bonus on recycling. In other words, just a double, not a double-double.
T & C
Spend $50 on HP ink in one transaction, earn 20% back in Staples Rewards® and earn $4 back in Staples Rewards® per ink cartridge recycled on same day of qualifying purchase for in-store purchases, or within 7 days for delivery purchases.
no details on limit in the ad though....