Duplicate parent components not (always) detected

[MichaelClerx]

Neither of these are detected:

<?xml version="1.0" encoding="UTF-8"?>
<!-- CellML Test Suite. https://github.com/MichaelClerx/cellml-validation -->
<!-- CellML 1.0, 6.4.3.2: A component may not appear more than once in a relationsip -->
<model name="component_ref_duplicate_child_1"
       xmlns="http://www.cellml.org/cellml/1.0#">
  <component name="A" />
  <component name="B" />
  <group>
    <relationship_ref relationship="containment" />
    <component_ref component="A">
      <component_ref component="B" />
      <component_ref component="B" />
    </component_ref>
  </group>
</model>

^ this is acceptable-ish, but not valid CellML

<?xml version="1.0" encoding="UTF-8"?>
<!-- CellML Test Suite. https://github.com/MichaelClerx/cellml-validation -->
<!-- CellML 1.0, 6.4.3.2: A component may not appear more than once in a relationsip -->
<model name="component_ref_duplicate_child_2"
       xmlns="http://www.cellml.org/cellml/1.0#">
  <component name="A" />
  <component name="B" />
  <component name="C" />
  <group>
    <relationship_ref relationship="containment" />
    <component_ref component="A">
      <component_ref component="B">
        <component_ref component="C" />
      </component_ref>
      <component_ref component="C" />
    </component_ref>
  </group>
</model>

^ This should definitely be detected

[jonc125]

A lot of these were deliberate decisions: we were deferring full model validation to other tools, and focusing cellmlmanip on the manip part. So it's not always guaranteed to do sensible things with invalid models.

[MichaelClerx]

It'll still need to be fixed somehow