Duplicated page numbers for last two pages after scan and merge

[Jawvig]

Describe the bug
The last two pages in the page thumbnails view have the same page number after "Scan and merge".

To Reproduce

1. Scan two pages with document feeder
2. Choose "Scan and merge into PDF" to scan the reverse of those two pages in the document feeder.
3. See the pages thumbnail panel in the right in the app. It has four pages with the last two both labelled "Page 3"

Expected behaviour
The final page should be numbered "Page 4"

Screenshots