Copying with the Option key

Try this:

Click on a file and start dragging it.

Before you release your mouse click, hold down the Option key. Notice
the green circle with the plus sign appear?

Now release your mouse click to drop the item. A duplicate has been
made and placed where you dropped it, the original file remains in
it's original position.

This technique can also be applied to text: Highlight some text, click
to drag it, then hold down the Option key and release the duplicated
text at the new location.