Hi, I'm trying to migrate from version 1.9.18 to version moodle moodle 2.0.9. My Moodle was originally created in version 1.5 and was updated without problems until version 1.9.18. I've done everything recommended to migrate to version 2.0. Running scripts to see that the Default and NOT NULL fields are well, deleting the log table etc and I get the following error, this error is deleting or not the log table and running scripts or not to do a check of the base data integrity of null or not null fields. I hope I can help.
the error is as follows

Migrated user icons 3926/3926.
100%
Error de ejecución DDL sql

Más información sobre este error
Debug info: Duplicate entry '2003-forum_displaymode' for key 'mdl_userpref_usenam_uix'
CREATE UNIQUE INDEX mdl_userpref_usenam_uix ON mdl_user_preferences (userid, name)
Stack trace:

line 397 of /lib/dml/moodle_database.php: ddl_change_structure_exception thrown
line 669 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
line 88 of /lib/ddl/database_manager.php: call to mysqli_native_moodle_database->change_database_structure()
line 75 of /lib/ddl/database_manager.php: call to database_manager->execute_sql()
line 892 of /lib/ddl/database_manager.php: call to database_manager->execute_sql_arr()
line 5437 of /lib/db/upgrade.php: call to database_manager->add_index()
line 1393 of /lib/upgradelib.php: call to xmldb_main_upgrade()
line 273 of /admin/index.php: call to upgrade_core()