vacuum analyze - cannot insert duplicate key...

medias=> vacuum analyze;
ERROR:  Cannot insert a duplicate key into a unique index


why i get this message, it's not the first time that i vacuum analyze my db.
but today, for a reason that i don't know, i got this message.

can you help me to find the error and fix it.
[PostgreSQL 6.5.3 on i686-pc-linux-gnu, compiled by gcc egcs-2.91.66]

i know, my version of postgresql is old. if you have a url who explain step
by step how to upgrade to 7.1, i will probably find the time to do it.

patrick