When you click the Underline button on the Formatting toolbar, Excel underlines the data in the selected cell or range. By default, that underline is a single line. If you need a double underline, you probably select the Cells command from the Format menu, click the Font tab, and then choose Double from the Underline control (see picture 1). However, that's a permanent change. Excel will continue to display a double underline until you change it back, which is probably inconvenient for most of us.
 

Picture 1

To quickly apply a double underline without changing any Excel defaults, simply hold down the Shift key as you click the Underline button.