Try it yourself.Four live demos.

Root cause: both orders read the stock level before either one saved, so both saw 1 and both went through.

var stock = GetStock(order.Sku);
stock.Qty -= order.Qty;   // no lock, no check
Save(stock);

The fix: lock the stock row and check the quantity inside one transaction, so the second order waits and then sees the real level.

using var tx = db.BeginTransaction();
var stock = GetStockForUpdate(order.Sku, tx); // row lock
if (stock.Qty < order.Qty) throw new OutOfStockException();
stock.Qty -= order.Qty;
Save(stock, tx);
tx.Commit();