Not having duplication is better for revenue because having duplicate content fill up your feed because lots of your friends re-shared something is bad UI. I'm not really that much more likely to click on something after the first time I'm shown something, and I start to get actively pissed off at it after a few unnecessary repetitions (this was a big complaint about the spam coming from Facebook games in the early days). Spamming up the feed with duplicate content means that for the same time spent browsing, someone sees fewer pieces of original content, meaning their average click rate is less, and yes, that costs revenue.

But fundamentally duplication costs revenue because it's a bad UI and bad UIs piss off users, leading them to use your product less.