The uuid module uses a single global buffer for storing random values 
obtained from the system. This can (and does) cause non-uniqueness of 
generated id's when using the uuid4 function in a multithreaded program.

The following snippet shows the problem - _buffer is the global buffer:

    # When the system provides a version-4 UUID generator, use it.
    if _uuid_generate_random:
        _uuid_generate_random(_buffer)
        return UUID(bytes=_buffer.raw)

It looks like the bug is already fixed in Python trunk:

def uuid4():
    """Generate a random UUID."""

    # When the system provides a version-4 UUID generator, use it.
    if _uuid_generate_random:
        _buffer = ctypes.create_string_buffer(16)
        _uuid_generate_random(_buffer)
        return UUID(bytes=_buffer.raw)
    ...

Changeset: r67318. The changeset was not related to this issue, but 
#4363 (Make uuid module functions usable without ctypes).

Yes, _buffer is not longer global.

Thanks for the report!