The first one seen; turn on “Keep the last one” to keep the last. “Save the removed rows” keeps the ones taken out as a file of their own.

Lists over 50,000 rows?

The free tier stops at 50,000 rows; the Pass opens more.

Clean a mailing list of duplicates

"Taro@example.com" and "taro@example.com " are the same person. Normalise, then count, and the duplicates show.

Drop the CSV on "Remove duplicates" and pick the email column as the one that tells rows apart.
Use the preset "Loose (ignore spaces and case)".
Check how many rows went and save.
To share it, continue to "Mask personal data" from "Continue" in the bar.

Which row is kept?: The first one seen; turn on “Keep the last one” to keep the last. “Save the removed rows” keeps the ones taken out as a file of their own.
Lists over 50,000 rows?: The free tier stops at 50,000 rows; the Pass opens more.

The CSV is never uploaded; everything runs in this browser.