# Pt-duplicate-key-checker **URL:** \ **Category:** Percona Toolkit\ **Created:** [March 27, 2024, 9:25am UTC](https://forums.percona.com/t/pt-duplicate-key-checker/29380 "2024-03-27T09:25:53Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Edward\_Hibbert](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/edward_hibbert/32/993_2.png) [@Edward\_Hibbert](https://forums.percona.com/u/Edward_Hibbert)\ **Post date:** [March 27, 2024, 9:25am UTC](https://forums.percona.com/t/pt-duplicate-key-checker/29380/1 "2024-03-27T09:25:53Z")
Suppose I have an index X on column A which is used as a constraint in a relation. And I have an index Y on columns A,B. Then pt-duplicate-key-checker shows up index X as a redundant index. But I don’t think it is, is it? It’s required for the constraint. ---
**Author:** ![matthewb](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/matthewb/32/34_2.png) [@matthewb](https://forums.percona.com/u/matthewb)\ **Post date:** [March 27, 2024, 3:13pm UTC](https://forums.percona.com/t/pt-duplicate-key-checker/29380/2 "2024-03-27T15:13:05Z")
> [@Edward\_Hibbert](#): > > which is used as a constraint in a relation I’m assuming your constraint is a foreign key? Foreign Keys can use a multi-column index for their constraint reference. Example: INDEX (A, B); FOREIGN KEY (A) REFERENCES tableFoo (id); So yes, even with FKs, (A) by itself is still duplicate as (A, B) can be used by the FK. ---
**Author:** ![svetasmirnova](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/svetasmirnova/32/21_2.png) [@svetasmirnova](https://forums.percona.com/u/svetasmirnova)\ **Post date:** [April 7, 2024, 10:32am UTC](https://forums.percona.com/t/pt-duplicate-key-checker/29380/3 "2024-04-07T10:32:31Z")
This is known bug, reported at [[PT-1414] - Percona JIRA](https://perconadev.atlassian.net/browse/PT-1414)