Is it possible to overlap two identical Uv Islands in order to create one complete mesh? EDIT 

MONKEY FILE



My question is related to all the meshes that have been created with the modifier mirror. 

Let's take an easy example: 



This mesh can be divided in 2 parts, PART A &amp; PART B: equal shape.

In order to optimize the space for my Normal map

1) i delete the PART A of my low poly / high poly

2) In this way, having only PART B, i unwrap PART B and bake it with the high poly

the results is this one; not so good but it's not the point of my question



3) i mirror my mesh in order to create again my PART A and i have now this one



4) as you can see the Uv map of PART A overlap, in the same position for each island, the uvmap of PART B  and then they share the same portion on normal map




When i watch the final render however i saw this result



There is a line that cut my mesh in the half. Is it possibile to fix this problem? Does this prove that it's impossible to overlap two identical uv maps?
Have half of your mesh with UVs done.*

Bake your stuff.
Mirror your model.



Your UVs will be overlapping but not at the time of baking. If you bake while overlapping Blender will get confused and want to put two images (even though they're exactly the same, Blender doesn't know that) at the same place and that's why it comes out weird.

*If you already have your UVs overlapping, applied the Mirror modifier and rather not delete half of the mesh to mirror it again, then you can select one of the halves' UVs, scale them super small and put them in a corner, like this:

Bake your stuff and then ctrl + Z so that the UVs are overlapping again, but your stuff is already baked.

Short answer: Overlapping only causes problems while the baking process happens. Bake only half of your model and then overlap the UVs.