SharePoint: how to have a unique restriction in the list?

Question

In SQL, it is possible to have fields that cannot contain duplicate data.

How is this possible in SharePoint?

Suppose you have a list containing values ​​of type

ORANGES
APPLES 
PEARS

How do you guarantee that the user cannot log in

ORANGES
APPLES 
PEARS
ORANGES

thanks

Answer 1

This cannot be done out of the box. But there are such community solutions:

• SPListConstraints (free but not open source)
• Unique column policy (open source, original solution from Scot Hiller )

Answer 2

It looks like it can be done in 2010

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