EnforceUniqueValues attribute on Field definition in Schema.XML not working, what am I doing wrong?

Question

I have the following field definition in the Schema.XML file for a custom list definition. Per the documentation setting EnforceUniqueValues and Indexed to true should enforce a uniqueness constraint on the field, but I am able to add duplicate values across list items through the UI.

<Fields>
      <Field Type="Text" 
             DisplayName="Client" 
             Required="TRUE" 
             EnforceUniqueValues="TRUE" 
             Indexed="TRUE" 
             MaxLength="255" 
             ID="{fa564e0f-0c70-4ab9-b863-0177e6ddd247}" 
             SourceID="{$ListId:Lists/Client;}" 
             StaticName="Title" 
             Name="Title" 
             ColName="nvarchar4" 
             RowOrdinal="0" />    
</Fields>

Am I doing something wrong here? Is there somewhere else I need to set this attribute?

Answer 1

Have you tried this for other fields than Title?

The spec for SPField.EnforceUniqueValues states that

You cannot set this property to true for a built-in field or for a site column.

Not 100% that this is valid for the declarative approachs for custom schemas, but test with another field "test" of type text and you will find out...