This is due to 546 people having the same URL: https://en.wikisource.org/wiki/A_Biographical_Dictionary_of_Ancient,_Medieval,_and_Modern_Freethinkers
Unfortunately, the source doesn't have more specific anchors.
Result: all items e.g., https://www.wikidata.org/wiki/Q309704 have the first person appearing in that page as the preview.
Description
Description
Event Timeline
Comment Actions
There is no way to fix this, due to the lack of specific pointers in the source.
It probably makes more sense to dismiss the small set of affected items.