To quickly create copies of an item (structure, Unit or Object) simply hold down the CTRL key before clicking the item...
To create lots of copies, select all the items, press CTRL+C then press CTRL+V. A new selection appears, and you can now drag these to where ever you need them..
If you place several items (of the same type) onto the same location, only one item will actually be available during the game. For example, if you place several levels of Galaxite onto the map, the Bullpup will only mine one.
Galaxite comes in various shapes and sizes. Only the size matters, as this is related to the number of credits they can be converted into...
Large : 500 credits
Medium : 300 credits
Small : 100 credits