The code looks like this:

[Serializable]
public class Hierarchy : Structure
{
    [Field(Indexed = true, Nullable = true)]
    public Guid? RootId { get; set; }
}

During domain build I get this error:

Xtensive.Orm.DomainBuilderException: Index with name 'IX_Hierarchy_RootId' is already registered.
at Xtensive.Orm.Building.Builders.TypeBuilder.BuildNestedFields(BuildingContext context, FieldInfo source, FieldInfo target, IEnumerable`1 fields)