Currently, ProofreadPage prevents the saving of Index: page with two links to the same page in the Page namespace.
This is done to support old Index pages that do not rely on a PDF/DjVu and builds their pagination from the Page: pages links.
However, this constraint is currently enforced for all Index: pages, even the ones that do not rely on links to Page: pages to build the pagination.
It might be nice to lift this constraint.