To duplicate a layer on your Canvas, hold down ⌥ and drag the layer. It’ll keep the original in its place.
You can also press ⌘D to duplicate a layer. However, where the duplicate appears will depend on what preferences you’ve set.
After duplicating a layer or group, you can press ⌘D to create a duplicate that’s at exactly the same offset as your first duplicate.
Holding ⇧ when using either ⌘D or the Edit > Duplicate menu option will place the new layer below the original layer (by default, duplicates are placed above the original layer).