If you are using a through the lens metered flash you can set the flash to fire either when the 1st curtain opens or when the 2nd curtain opens. Since the shutter will stay open until the sensor has sufficient light if you set it to fire with the first curtain, the 2nd curtain will close almost intaneously giving a sharp image. If you set it to fire as 2nd curtain the 1st curtain will open allowing light to the sensor, the flash will fire, the 2nd curtain will then operate. This gives a double image one for the first curtain opening and one for when the flash fired.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html