There are several ways to duplicate objects.

Keyboard

Select an object and press Ctrl + d.

Using the Mouse

Select the object. Then press the Alt key and start moving the object. You will see a blue frame appear.

Using the Mouse (Deluxe)

Select the object. Then press the Alt key and start moving the object. Now you can use the arrow keys to determine how many row and column copies you want to create and drag it all out as a grid.

Using a Dialog

You can also use the dialog "Multiple Copy" to create copies.