Error when upgrading from OpenMRS 1.6 to 1.9

?There was an error while updating the database to the latest. file: liquibase-update-to-latest.xml. Error: Migration failed for change set liquibase-update-to-latest.xml::20110825-1000-creating-providers-for-persons-from-encounter::raff:?
??? where user.user_id is not null and encounter.provider_id not in (select person_id from provider): Duplicate entry 'f419a7c0-641b-4ab1-81b5-7bfe18d18611' for key 'uuid':???
??? inner join encounter as encounter on encounter.provider_id = person.person_id???
??? select distinct person.person_id, user.system_id, 1, CURRENT_TIMESTAMP, user.retired, user.retired_by, user.date_retired, user.retire_reason, person.uuid???
??? on person.person_id = user.person_id???
??? from users as users group by users.person_id) as user ???
??? from person as person???
Unable to update the database. See server error logs for the full stacktrace.
??? Caused By: Duplicate entry 'f419a7c0-641b-4ab1-81b5-7bfe18d18611' for key 'uuid'???
??? Caused By: Error executing SQL insert into provider(person_id, identifier, creator, date_created, retired, retired_by, date_retired, retire_reason, uuid)???
??? Reason: liquibase.exception.DatabaseException: Error executing SQL insert into provider(person_id, identifier, creator, date_created, retired, retired_by, date_retired, retire_reason, uuid)???
??? left join (select max(users.user_id) as user_id, users.system_id, users.person_id, users.retired, users.retired_by, users.date_retired, users.retire_reason ???