wow, that was a long time, the thumbnails finally finished rebuilding 3 hours later.
I also saw this option and wanted to know exactly what it would do should I ever choose it.
"Check here to scan for duplicate entries for same photo"
What happens if it finds duplicate entries? Does it show them to me side by side and who they belong to?
I would hope that we wouldn't find different people with the same image upload, but you never know. Stranger things have happened.