How to Find Duplicates Between Two Lists

Guide · Updated June 2026

Two different kinds of “duplicate”

“Find duplicates” usually means one of two things: items that show up in both of two separate lists, or items that repeat within a single list. Each needs a different tool.

Duplicates between two lists = the intersection

If you have two exports — say, last month’s customer list and this month’s — and want to know which records exist in both, paste one into List 1 and the other into List 2, then read the Intersection (A ∩ B) result. That set is exactly your duplicated/shared records. See Union vs. Intersection vs. Difference if the terminology is new.

Duplicates within a single list

If you just have one messy list with repeated entries, paste it into either panel and use the Cleanaction — it removes duplicate lines according to your current match rules (case sensitivity and whitespace trimming both affect what counts as “the same”). The item-count badge updates immediately so you can see how many entries were removed.

Case sensitivity changes the answer

With case sensitiveoff (the default), “Alpha” and “alpha” are treated as duplicates of each other. Turn it on when case actually matters — product SKUs, order IDs, or anything where “ABC-1” and “abc-1” should be counted separately.

Whitespace is a common false negative

“Alpha” and “Alpha ” (with a trailing space) look identical but won’t match as text. Keep Trim spaces on unless you have a specific reason to compare raw, untrimmed strings.