Summary says it all really. But to give a little more detail:

In plugin.py in install_plugin there is a line:

same_file = os.path.samefile(path, dest) if dest_exists else False

and when this is executed under Windows you get:

Traceback (most recent call last):
File ".\picard\ui\options\plugins.py", line 133, in open_plugins
self.install_plugin(path)
File ".\picard\ui\options\plugins.py", line 147, in install_plugin
self.tagger.pluginmanager.install_plugin(path, dest)
File ".\picard\plugin.py", line 192, in install_plugin
same_file = os.path.samefile(path, dest) if dest_exists else False
AttributeError: 'module' object has no attribute 'samefile'