Unfortunately we can still not reproduce the problem. However, an inverse problem could be reproduced before 2019.3.12, after importing the latest unitypackage over an existing installation: errors stating double definition of class Animation etc were stated. So it definitely looks like a Unity cache related problem.
The best guess I currently have is it might be the two package.json files which have recently been added to allow using spine-unity with git and the Unity package manager. Perhaps some cached files of the Unity project are incorrectly updated because of it.
I have now removed the two package.json files from the unitypackage, a new 3.8 unitypackage is availabe for download here as usual:
Spine Unity Download
You could have a try whether removing the old Spine and Spine Examples directories and importing the latest package resolves your problem. Please let us know whether this resolves your problem.
Sorry again for the trouble and thanks for reporting!
