"It's a woman's prerogative to change her mind," says the old expression. Well, that goes for men too. And traders (who we consider a third category).
If you've placed an order to buy shares of stock when it hits a specific price, and then decide to buy it today at the current market price, you need to officially cancel the original one with a cancel former order. Then you can replace it with a new buy order to purchase the market-priced shares. The same goes for sell orders; you need to submit a cancel former order if you change your mind.
Let's say Dude Jones has an outstanding order to buy 100 shares of Don't Cancel Me Inc. when it goes down to $20 a share. After two months, the Dude gets tired of waiting and decides to buy it at the current market price of $22 a share. If he doesn't submit a cancel former order for the $20 a share order, he will end up with a duplicate order for that stock.
