How to remove duplicated vertices at runtime

I took a look at this post https://answers.unity.com/questions/1382854/welding-vertices-at-runtime.html to weld vertices.

Answer of user @imaxus is right but there is a problem with lights and normals.

But answer of @Bunny83 does not work. Why? Can someone explain why?

I used the @imaxus's code. But there is a issue with lighting and normals. How to fix?

You can see the normal and lighting issue in the image when I am using the code of @imaxus.

But @Bunny83 's codes does not work? HELP Whats wrong with your code @Bunny83