Allow Only One

I love the concept of this module. For your content types you can define rules for checking that content is unique, each rule checking a field or a combination of fields.

That said, I found the implementation a little strange. I would expect to manage these rules in a tab for the content type, but instead you add them as fields, and then it's suggested to move the rule close to the field(s) it examines.

Also, as of this writing the module doesn't work with multivalued fields, or fields that have compound values, such as link fields that contain both a URI and (optionally) link text.