I now avoid getting duplicates of Shared Cars because I never buy them permanently.
I buy all available cars on PSP1, share them to PSP2, reset PSP1 (no autosave), and then share them back. So I get all my shareable cars for free. Every now-and-then while going quickly I accidentally duplicate a colour and car, but it doesn't matter now, because it will go away after the reset anyway. (So if you set autosave off, and you accidentally duplicate a car, one option might be to go back to your most recent save if you notice in time).
Regarding getting duplicate cars by share, that's not true, but you can get duplicate cars by trade, and with little (no?) warning.
Shareable cars for free. Tradeable cars for the cost of the most convenient cheap Tradeable around.