How do I make value of a profile field unique?

Hi,

I have a situation where I need to maintain a profile fields value as unique.
e.x. a field by name 'Internal Customer No' field should have a unique value value for all the customers i add.

I found that all the values for any of the profile fields go to profile_values table in db. Where value for all the fields on my page are saved in a single field called 'value'.

How do I make the value of this field unique for only one single field on my page???

Any help.

Regards,
P.