Is it forbidden for a man to sell what his brother has already sold, or is the first sale valid?

It is not permissible for a Muslim to sell over the sale of his brother or to buy over his purchase, due to the enmity and hatred that this causes among Muslims. However, if a Muslim violates this and does so, is the sale valid or not? There are two opinions: The first opinion is that the sale is invalid, which is the madhhab of the Hanbalis. The second opinion is that the sale is valid, but sinful, which is the madhhab of the Shafi'is.