[Valloric]

Originally Posted by kovidgoyal

Certainly, the spec should specify that (title, type) must be unique. From the perspective of Sigil, I would assume that the spec does specify that.

I'm not following you. The spec doesn't even mention the "title" attribute in the guide section (except in their small example, which is non-normative, so it in effect doesn't exist), and the only conclusive proof that it is indeed optional comes from the RELAX NG schema in Appendix A:

<define name="OPF20.reference-element">
  <element name="reference">
    <ref name="OPF20.optional-id-attribute"/>
    <attribute name="type">
      <text/>
    </attribute>
    <optional>
    <attribute name="title">
      <text/>
    </attribute>
    </optional>
    <attribute name="href">
      <text/>
    </attribute>
    <ref name="OPF20.reference-content"/>
  </element>
</define>

So I don't see how it can be assumed from the perspective of Sigil that the spec specifies that (title, type) is a unique tuple.