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
2. Change existing column to unique values.
If existing column contain duplicate values You must remove all the duplicate values to a unique column and then set to unique values.
3. Change the data type of the unique column
Change the column to allow duplicate values, remove the index, change the column to the other supported data type, and then make the column unique
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
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