Scratch

There are two ways you can do it.
The first way is if you're trying to copy the script and keep the script in the same sprite.
  To do that, you would have to right-click the script you want to duplicate and select   
  duplicate.
The second way is to copy a script from a certain sprite and transfer it to another sprite
  You would drag and drop the script next to the sprite you want it to go to until a box
  appears around the sprite you want. Let go of the script and it would go back to it's
  original position while also duplicating it into the other sprites script area.

Highlight text, press Ctrl+C to copy, and Ctrl+V to paste.

You can also use Alt+C, Alt+X, and Alt+V, and if you every try making a mod, that will come in very helpful.