I seem to remember, way back in the olden times of yore, before all the updates and exploration bonus's, back when explorers were scared of cracking a canopy on a bad jump into a tight binary, when it took an age to plot a full 100ly jump route and every new system was branded "here be dragons", that back then FDev had said that pilots attempting to sell scans of previously scanned systems would suffer depreciating returns on the data any way.
I must have read or heard it somewhere because I remember, back in the early days, that whilst there was the obvious rush to get out there and get your name on something, there was also a drive to get out there and scan stuff because the more people that sold the same data, the less it would be worth anyway.
Surely, if you were to want to nerf income from exploration (and don't misunderstand me, I most definitely do not agree with that sentiment) then the best option would be to implement this feature.
In that way, first and early scans of a system would still reward the discovering players well, but as concurrent players returned data for the system, the data value would decrease in a natural way.
If nothing else, it'd eradicate the Road to Riches issue - there's no point following the same route as a couple of thousand other players if your data is going to be worth progressively less and less as more commanders complete the route before you.