Thanks for the additional info. The animation name FINAL/IDLE is the problem then, as a filename can't contain slashes, thus creation fails. I'm afraid I see no clean workarounds for this. You could duplicate another animation as a placeholder, rename the animation (to e.g. final_idle.anim) and then open the asset in a text editor and change the m_Name property to your target animation name FINAL/IDLE. However, every time you reimport the folder or asset, the name is automatically "corrected" to match the filename, so this is annoying unfortunately and would require a version control "revert changes" (git checkout) to undo these automatic undesired changes.
Another workaround would be to place any animations that need duplicates not within a folder in Spine.
A different workaround would be to edit the spine-unity source code accordingly to replace the slash "/" character in any read animation name with another character, such as "_" or the like.