One of the problems you might have is that you want to restrict your Portal Users/Admins to enter the same value twice for a given attribute. Examples might be the account name or employee id. Jorge has a nice article on how to configure this: http://blogs.dirteam.com/blogs/jorge/archive/2009/12/10/checking-uniqueness-of-an-attribute-in-fim-2010-during-the-create-process.aspx
One of the remarks was that this only works for Resource Control Display Configurations (RCDC’s) in create mode. However on the forum (http://social.technet.microsoft.com/Forums/en-US/ilm2/thread/cc51ca7a-908c-40bf-ae10-f47711dd321b) I read that it would also work in edit mode. So I went ahead and tested. After clicking a user and trying to alter the Account Status (a non-unique enforced attribute) I get the alert for the Account Name attribute (unique-enforced in the RCDC). So it seems that however when only changing one attribute in edit mode, all attributes are checked anyhow. I guess that’s the reason why using the “UniquenessValidationXPath” is not supported/does not work for RCDC’s in edit mode.
Conclusion: RCDC’s cannot enforce the uniqueness of an attribute in edit mode.