Import fails with unique constraint violation

We've tried to export our Confluence site from version 1.2.3 and import it into a clean 1.3.1 installation. The import fails with unique constraint violation.

The constraint SYS_C0016204 is the primary key constraint of CONTENT.CONTENTID.

18.1.2005 9:36:30 net.sf.hibernate.JDBCException <init>
SEVERE: could not insert: com.atlassian.confluence.user.PersonalInformation#3961
java.sql.BatchUpdateException: ORA-00001: unique constraint (CONFLUENCE.SYS_C0016204) violated
at oracle.jdbc.driver.DatabaseError.throwBatchUpdateException(DatabaseError.java:367)
at oracle.jdbc.driver.OraclePreparedStatement.executeBatch(OraclePreparedStatement.java:8739)
at org.apache.commons.dbcp.DelegatingPreparedStatement.executeBatch(DelegatingPreparedStatement.java:231)
at net.sf.hibernate.impl.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:54)
at net.sf.hibernate.impl.BatchingBatcher.addToBatch(BatchingBatcher.java:34)
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:468)
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:442)
at net.sf.hibernate.impl.ScheduledInsertion.execute(ScheduledInsertion.java:29)
at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2418)
at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2371)
at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2240)
at com.atlassian.confluence.importexport.impl.BackupImporter.doUpgrades(BackupImporter.java:142)
at com.atlassian.confluence.importexport.impl.BackupImporter.doImport(BackupImporter.java:87)
at com.atlassian.confluence.importexport.DefaultImportExportMana18.1.2005 9:36:30 net.sf.hibernate.JDBCException <init>
...
18.1.2005 9:36:30 net.sf.hibernate.impl.SessionImpl execute
SEVERE: Could not synchronize database state with session
net.sf.hibernate.JDBCException: could not insert: com.atlassian.confluence.user.PersonalInformation#3961
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:478)
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:442)
at net.sf.hibernate.impl.ScheduledInsertion.execute(ScheduledInsertion.java:29)
at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2418)
at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2371)
at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2240)
at com.atlassian.confluence.importexport.impl.BackupImporter.doUpgrades(BackupImporter.java:142)
at com.atlassian.confluence.importexport.impl.BackupImporter.doImport(BackupImporter.java:87)
at com.atlassian.confluence.importexport.DefaultImportExportManager.importAs(DefaultImportExportManager.java:48)
...