Full traceback (I modified venv/__main__.py):

haypo@smithers$ /opt/py34/bin/python3 -m venv --upgrade ENV
Error: [Errno 17] File exists: '/home/haypo/ENV/lib' -> '/home/haypo/ENV/lib64'
Traceback (most recent call last):
  File "/opt/py34/lib/python3.4/runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "/opt/py34/lib/python3.4/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/opt/py34/lib/python3.4/venv/__main__.py", line 6, in <module>
    main()
  File "/opt/py34/lib/python3.4/venv/__init__.py", line 438, in main
    builder.create(d)
  File "/opt/py34/lib/python3.4/venv/__init__.py", line 82, in create
    context = self.ensure_directories(env_dir)
  File "/opt/py34/lib/python3.4/venv/__init__.py", line 147, in ensure_directories
    os.symlink(p, link_path)
FileExistsError: [Errno 17] File exists: '/home/haypo/ENV/lib' -> '/home/haypo/ENV/lib64'

It looks like a regression introduced by the issue #21197.

Attached patch should fix it.