Files that have already been uploaded can be copied either individually or together:
Click on one file or multiple files which you wish to copy. You can do this by pressing and holding down the left mouse button and move it over the files that you want to copy. You can release the mouse button after you finished selecting the files.
When you right click, a window will open and you can select "copy". Next open the destination folder. Click the right mouse again in the folder and choose "insert" in the window.