Find Duplicates Between Two Lists

Guide · August 24, 2026

Finding duplicates between two lists is one of the most common data tasks. You have two lists of items — emails, usernames, product IDs, file names — and you need to answer questions like: which items appear in both? Which are only in the first list? Which are only in the second?

DiffLab's List Compare tool answers all of these at once by computing five set operations: intersection (in both), two differences (only in A, only in B), union (all unique), and symmetric difference (changed items only).

Common use cases

Find who doesn't follow you back

Export your "following" list and your "followers" list from a social platform. Paste following into List A and followers into List B. The Only in A result shows accounts you follow that don't follow you back.

Find duplicate entries across two datasets

Paste the first dataset's key column into List A and the second's into List B. The In Both result shows which keys appear in both datasets — your duplicates.

Find new and removed items between snapshots

Paste last week's inventory list into List A and this week's into List B. Only in A shows items that were removed. Only in B shows items that were added.

Find missing records

Paste a reference list (e.g., all expected product SKUs) into List A and an actual list (e.g., SKUs found in the database) into List B. Only in A shows SKUs that should be in the database but are missing.

The five set operations explained

Only in A (A - B): Items present in List A but not in List B. These are entries unique to the first list.
Only in B (B - A): Items present in List B but not in List A. These are entries unique to the second list.
In Both (A ∩ B): The intersection — items that appear in both lists. These are your duplicates or overlaps.
Union (A ∪ B): All unique items from both lists combined. Duplicates within and across lists are collapsed.
Symmetric Difference (A Δ B): Items that are in one list or the other, but not in both. This is the set of all changed items.

Note: The List Compare tool treats each line as a distinct item. If your list has internal duplicates (the same value appears twice in List A), they are collapsed into one. The tool compares sets, not multisets. If you need to preserve duplicate counts, use the Text Compare tool in line mode instead.

Note: Comparison is case-sensitive by default. Apple and apple will be treated as different items. If you want case-insensitive comparison, normalize your data (e.g., convert to lowercase) before pasting, or use the case-sensitivity toggle.

Find duplicates between your lists now.

Open List Compare