Authors sorted randomly

I experience this both with 6.x-1.10 and 6.x-1.x-dev (27 of January download) with Drupal 6.15 (imported 399 references from Endnote X3).

There seems to exist some problem with the sorting of the database by using the "author" criteria.

For instance with ?sort=author&order=asc I get this (an author with "P" is appears amongst authors with "A"):

A
Ackermann, E. (2001).  Piaget's constructivism, Papert's constructionism: What's the difference?. Future of learning group publication.
Adorno, Theodor, & Stephen Crook (2002).  Adorno : the stars down to earth and other essays on the irrational in culture. Routledge classics. vi, 239 p..
Al-khamayseh, Shadi, Agnieszka Zmijewska, Elaine Lawrence, & Gordana Culjak (2007).  Mobile learning systems for digital natives. Proceedings of the sixth conference on IASTED International Conference Web-Based Education. 2,

P
Postman, Neil (2007).  O Fim da Educação. Redefinindo o valor da escola..

A
Alexander, Bryan (2004).  Going Nomadic: Mobile Learning in Higher Education. EDUCAUSE Review. 39, 28-35.

If I click the name of the P author I get (the same key/entry appears 2 times and although "Export" correctly states "2 results" If I export the resulting files has a repeated entry)
biblio/author/445?sort=author&order=asc:

Postman, Neil (2007).  O Fim da Educação. Redefinindo o valor da escola..
Postman, Neil (1993).  Technopoly : the surrender of culture to technology. xii, 222 p..
Postman, Neil (2007).  O Fim da Educação. Redefinindo o valor da escola.

.

But when I sort the reults by "year", "title" or "type" I instantaneously get it right (biblio/author/445?sort=year&order=asc), e.g.:

1993
Postman, Neil (1993).  Technopoly : the surrender of culture to technology. xii, 222 p..

2007
Postman, Neil (2007).  O Fim da Educação. Redefinindo o valor da escola.

So the issue seems restricted to sorting by "author" alone. Any ideas on what may cause this?

Thanks