Enforce Unique Values on a Column

Scenario/Problem: You want to make sure no two list items in a list or library have the same value in a column. For example, you want to make sure no two files have the same title or no two tasks have the same title.

Solution: The option to enforce unique values is available for some column types (refer to Figure 7.10), including the single line of text, number, choice, and lookup. Selecting Yes in this option tells SharePoint to make sure that users do not enter the same value in two different list items.