Much like required fields on a collection, make a "unique" option.
This will restrict other items in the same collection from having that same value in the respective field.
For example: My collection is products. I don't want there to be 2 product with the same SKU (field). When I setup the product collection I'd like to be able to enable "Unique" on the SKU field.
This way, there can't be 2 product records with the same SKU.