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This seems to be a bug in ntpath.samefile, in particular in this code:
# determine if two files are in fact the same file
try:
 # GetFinalPathNameByHandle is available starting with Windows 6.0.
 # Windows XP and non-Windows OS'es will mock _getfinalpathname.
 if sys.getwindowsversion()[:2] >= (6, 0):
 from nt import _getfinalpathname
 else:
 raise ImportError
except (AttributeError, ImportError):
 # On Windows XP and earlier, two files are the same if their absolute
 # pathnames are the same.
 # Non-Windows operating systems fake this method with an XP
 # approximation.
 def _getfinalpathname(f):
 return abspath(f)
def samefile(f1, f2):
 "Test whether two pathnames reference the same actual file"
 return _getfinalpathname(f1) == _getfinalpathname(f2)
Python2 doesn't have ntpath.samefile and shutil then falls back to comparing "os.path.normcase(os.path.abspath(src))" with the same transformation of dst.
On XP _getfinalpath doesn't call os.path.normcase, hence it doesn't notice that "a" and "A" refer to the same file (on all common NT filesystems)