I’ve included an image. I believe the UVs are never outside the 0-1 space when exclusively letting blender build a lightmap for you. I don’t usually triangulate a mesh before import, I just did that with this mesh after the errors for extra precaution. The UV channel “00” may have mirrored/overlapping UVs but that’s the texture channel, the LM channel, to my knowledge, has none of those things.