Recently I ran into the issue that the SharePoint Online search was not returning all expected search results – or so it seemed.
To do a test on a site with many items, I uploaded around 7000 documents, all with a different file name but the contents were identical.
As it turns out, SharePoint by default hides the duplicates -and doesn’t show you the ‘Show Duplicates’ link, which is hidden in the popup window for a search result anyway.
The effect was that while I was expecting 7000 results from a search, I only got 1 hit.
If you want to see all your results, you would ideally want to switch off some option on the results web part, to tell it to stop hiding duplicates. Unfortunately that option is not available on the web part UI (something Microsoft should really have put in there IMHO).
Luckily there are some workaround, and you can also enable the ‘Show Duplicates’ link, although I think that solution is far from ideal.
You can either change the TrimDuplicates setting in the web part code (by exporting the web part, changing the setting and re-uploading it), or change the query in the web part settings to make it GroupBy the property DocumentSignature.
Here are the relevant blog posts describing all this in more detail: