In Marvin JS, there are many ways to multiply your structures.
Inside the application
Multiply the molecule with Ctrl+Drag :
You can move a copy of the selected structure by pressing the Ctrl button and dragging the selected structure. When you do this, a "+" sign appears beside the cursor. A copy of the selected structure is placed on the canvas when you release the left button, and the original object remains selected, so it can be multiplied several times.
Between Marvin JS and other applications*
Select what you like to copy, then press the keyboard shortcuts.
*Direct copy-paste between Marvin JS and other applications is available in MRV or MDL Molfile (V2000) format without web services. If web services are available, direct copy-paste between Marvin JS and other applications are possible in every text formats.