To un-delete objects, click over them in the Organizer window.
The duplicated object appears on top of the original object. Press H to use the transform handle to move it away.
Multiple duplications with transformation (Smart Duplicate)
The first duplicated object appears on top of the original object. Press H to use the transform handle to alter the position, rotation, or scale of the duplicate.
Repeating duplications with transformations is an easy way to equally space copies of an object, or create patterns of objects in the scene.
You can undo your actions at any time. Press Ctrl+Z, or select Edit > Undo.
You may need to undo several times to get back to the desired state.