With my (and everyone else at my companies) setup with etrade, you get a 1099, and then later an addendum with the adjusted values. Turbotax by default tells you to enter the 1099 sales, and after entering and submitting each row, you see the chance to adjust the cost basis, which you must enter manually from the addendum. Perhaps other brokerages have a more elegant setup. If you don't adjust you will be double taxed. This would likely be obvious to you if you sold a lot of shares, but if you just did some small sales, I can easily see someone accidentally double taxing themselves.