Duplicate title
What it means
Several indexable pages share the same title. Search engines use it to tell pages apart, so duplicates suggest the pages are interchangeable, and make the ones that do get indexed compete with each other.
How to fix it
Give each page a title that describes what is unique about it. Templated titles are the usual cause: a paginated series needs the page number, a filtered listing needs the filter, and a product variant needs the variant. If two pages genuinely are the same, consolidate them and canonicalise one to the other instead.
Finding it in CrawlStat
After a crawl, open Issues and pick Duplicate title. Click Show all in Pages to filter the Pages table to the affected pages, or Export affected to hand the list to whoever owns the fix. Audit ID: title_duplicate.