To duplicate a layer on your Canvas, hold down ⌥ and drag the layer. The original layer will stay in its place.
Releasing ⌥ simultaneously or slightly before releasing your mouse (or trackpad) will duplicate the layers instead of moving them.
You can also press ⌘D to duplicate a layer. Where the duplicate appears will depend on whether you have the Offset duplicated layers option enabled, which you’ll find in Sketch > Settings > Layers.
After duplicating a layer or group with the offset option enabled, you can press ⌘D to create a duplicate that’s at exactly the same offset as your first duplicate.
By default, duplicates are placed above the original layer. Holding ⇧ when using either ⌘D or selecting the Edit > Duplicate menu option will place the new layer below the original layer.