If the same data about you is collected several times by several different companies, each individual data item it will of course be worth less even if only because there are now multiple competing companies trying to sell that data item harvested from you.

You see, unlike with creative works for which governments created artificial monopolies (which is what IP legislation is all about), data about you doesn’t have any such thing as Copyright limiting the copying of it (if it did, the copyright would naturally be yours, which would screw all those data harvesting businesses, which is why politicians would never ever make it so), so it acn be collected multiple times and copied at will unless in some kind of processed way.

If you think of it, it’s not by chance that Google’s collected private data isn’t sold directly but rather advertising services are sold on top of it: if the data never leaves the private-data-harvesting company’s system there won’t be tons of copies floating around. What’s sold is an ad service, rather than the data, so the collection of that data by other companies won’t devalueate Google’s profit from eit quite as much because they’re not selling the data but a service on top of it and those other companies won’t be able to sell quite the same service unless they reach Google’ breadth of spying on people.

However everybody and their dog is following that same strategy so everybody and their dog are reaching levels of spying on people close enough to Google’s that we still end up with enough competition on “ads over private data” and hence indirectly that lowers the value for that data.