Hi Klo Works,
As others have suggested above mirroring UVs is not the best option when using normal maps. There are ways around it but it requires editing your Normal Map in an image editing program like Photoshop. You can find a tutorial on Polycount (Link Here). This may be something you may choose to do or not. The easier method by far is to use unique uv space for each uv island of your model.
Another issue with your models is that you absolutely need a second UV channel for your lightmap. Especially when mirroring or overlapping any UV space. All the lighting for static meshes gets baked into a lightmap. Currently the mesh will have a bad lightmap due to overlapping UVs. If you’re not familiar with lightmapping you can find the tutorial at the link here.
I hope this helps answer some of the issues you’re seeing!
Tim