Error when two custom data sets extend the same thing

1. Change the 'Constituent Information''s used for to 'Contacts'.
2. Create a new custom data set that is used for 'Contacts'.
3. Try to edit the new custom data set; you get db error: Duplicate entry 'Contact' for key 'UI_name_extends'

This is because UI_name_extends requires that the used for is unique. If it's expected to be unique, then it shouldn't be possible to create two custom data sets that are used for the same thing; if two custom data sets should be able to extend the same thing then it shouldn't be unique.