In Unity 4, you could easily duplicate baked lightmapped objects and they would still share their lightmap with the original and save lightmap textures and size. In newer Unity versions however, if you try to duplicate a lightmapped object, the duplicate won’t have any lightmapping.
For more detail this asset from the Unity Asset Store: Click Here
This is a paid asset, but now you can download the “Duplicate with Lightmap” for FREE, Please keep in mind this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Duplicate with Lightmap v1.1
In Unity 4, you could easily duplicate baked lightmapped objects and they would still share their lightmap with the original and save lightmap textures and size. In newer Unity versions however, if you try to duplicate a lightmapped object, the duplicate won’t have any lightmapping.
With this Plugin, you can conveniently duplicate lightmapped objects with CTRL + SHIFT + D and reuse them – they keep the original objects lightmap, so you can easily save on the lightmap size and don’t have to rebake all the time.
When you re-bake your original GameObject, all the duplicates will also have the new lightmap information.