Get error when trying to specify "Implementation Child Ordinality" of 1

I have 2 content types: Requests and Implementations. There is a 1:1 relationship with them with the Request as the parent and the Implementation as a child. Here is how I have things configured:

1. Node types that can be involved in relationships: Requests and Implementations
2. For Requests, Parent Ordinality is None and Allowable Child Node types is Implementation
3. For Implementations, Parent Ordinality is One and Allowable Child Node types is null
4. On the Advanced Settings page, Implementation Child Ordinality for Request Parents is Any

I think I need to change Implementation Child Ordinality for Change Request Parents to One so that, when an Implementation has been linked to a Request, no further Implementations can be created from the Request and/or linked to it. However, when I select One from the list and click save, I get the following error message and the setting change doesn't stick:

user warning: Duplicate entry 'relativity_child_ord_change_request_project_impl' for key 1 query: INSERT INTO variable (name, value) VALUES ('relativity_child_ord_change_request_project_implementation', 's:1:\"1\";') in C:\Apps\EasyPHP2.0b1\www\drupal-5.3\includes\database.mysql.inc on line 172.

Anyone have an idea what might be going on? Thanks!