In SharePoint 2007 you couldn't enforce uniqueness on a column in a list or document library to ensure that the column would never have duplicate values.
Supported Column Types
Single line of text
Choice (single value)
Date and Time Lookup (multi-valued)
Person or Group (single value)
Unique column Limitations
1. Remove the index of a unique column.
You must change the column from a unique column to one that allows duplicate values first, and then remove the index
4. You cannot copy a file to a library by using the Open with Explorer command.
5. You cannot break inheritance of permissions from a list to a list item.
6. Item-level permissions for not allowed
7. If list that contains items with unique permissions.
8. List items with content approval enabled
9. Case-insensitive “Product 1” and “product 1”. This is because these two values are considered equivalent.
10. Lookup column supported uniqueness in this case means that more than one list item in the child list cannot look up to the child list cannot look up to the same list item in the parent list