If you have created a role in Moodle 1.9 called "Manager", the upgrade to 2.x will fail as it will try and create a new role called "Manager", and the name field of the roles table must be unique.

2 possible solutions would be:

1. Leave the current "Manager" role in place and use it for 2.x's purposes (if that wouldn't cause problems), or
2. Check for the existence of the "Manager" role at the start of the upgrade script, and tell the user they must rename it before the upgrade can continue.

This issue has been discussed before, but not completely solved. See related issues.