not double face or none

[old swallow]

Then more info is required

https://forum.unity.com/threads/faq-blocky-artifacts-appear-when-i-bake-a-scene-how-can-i-fix-them.951444/
Here's what causes texel invalidity

[limpid anvil]

k

Ill look at it

[old swallow]

z-fighting is caused by overlapping geometry, like this