Duplicate headings, coming from different wiki pages, have the same generated ID in the PDF export

When exporting multiple wiki pages to PDF there's a good chance that some of the headings are "duplicated" across the exported wiki pages. When this happens:

• the page number displayed on the table of contents is wrong (all occurrences of the "same" heading point to the print page where the first occurrence is)
• the link from the table of contents points to the first occurrence

This probably affects other content blocks that use generated identifiers.