When opening a tarfile for writing and adding several
files, some files end up being a hardlink to a
previously added tar member instead of being a proper
file member.

I attach a demo that demonstrates the problem. It
basically does:

tarfile.open('tarfile-bug.tar', 'w')
tar.add('tarfile-bug-f1')
tar.add('tarfile-bug-f2')
tar.close()

in the resulting tar, "tarfile-bug-f2" is a hard link
to tarfile-bug-f1, although both entries should be
proper files.

It works when the tarfile is close()d and opened again
in append mode between the two add()s, but that slows
down the process dramatically and is certainly not the
intended way.