In several graphics editors and on macOS generally throughout apps (Finder, Calendar, etc.), holding down the Option (Alt) key while dragging copies and positions the object.
In draw.io, one can currently achieve this by hitting ⌘D (or ⌘C+⌘V), moving the mouse to the duplicate object, and dragging it. Option+drag is easier.
In several graphics editors and on macOS generally throughout apps (Finder, Calendar, etc.), holding down the Option (Alt) key while dragging copies and positions the object.
In draw.io, one can currently achieve this by hitting ⌘D (or ⌘C+⌘V), moving the mouse to the duplicate object, and dragging it. Option+drag is easier.