>>The index MUST also be uniquely identifiable using a combination of the d tag's first value (usually containing the title), the pubkey, and the kind fields.<<

This is the part in the NKBIP-01 that pertains to the d tag. We could change the text to read

>>The index MUST also be uniquely identifiable using a combination of the d tag's values (at least including the first value, usually the title), the pubkey, and the kind fields.<<