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Your UNIQUE key is the combination of (slug, tag_check). NULL values are ignored and your insert query always writes NULL to tag_check column. CREATE TABLE `dupe_test` ( `id` int(11) NOT NULL AUTO_INCREMENT, `slug` varchar(50) NOT NULL DEFAULT '', `tag_check` tinyint(4) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `player_tag_check` (`slug`,`tag_check`) ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci; select * from dupe_test; +----+-----------+-----------+ | id | slug | tag_check | +----+-----------+-----------+ | 1 | hugh-jass | NULL | | 2 | hugh-jass | NULL | | 3 | hugh-jass | NULL | | 4 | hugh-jass | 1 | +----+-----------+-----------+ Add another... MariaDB [test]> insert into dupe_test (slug) values ('hugh-jass'); Query OK, 1 row affected (0.051 sec) MariaDB [test]> select * from dupe_test; +----+-----------+-----------+ | id | slug | tag_check | +----+-----------+-----------+ | 1 | hugh-jass | NULL | | 2 | hugh-jass | NULL | | 3 | hugh-jass | NULL | | 6 | hugh-jass | NULL | << NEW | 4 | hugh-jass | 1 | +----+-----------+-----------+ 5 rows in set (0.000 sec) Now try adding again but with tag_check = 1 then 2... MariaDB [test]> insert into dupe_test (slug, tag_check) values ('hugh-jass', 1); ERROR 1062 (23000): Duplicate entry 'hugh-jass-1' for key 'player_tag_check' MariaDB [test]> insert into dupe_test (slug, tag_check) values ('hugh-jass', 2); Query OK, 1 row affected (0.073 sec) MariaDB [test]> select * from dupe_test; +----+-----------+-----------+ | id | slug | tag_check | +----+-----------+-----------+ | 1 | hugh-jass | NULL | | 2 | hugh-jass | NULL | | 3 | hugh-jass | NULL | | 6 | hugh-jass | NULL | | 4 | hugh-jass | 1 | | 8 | hugh-jass | 2 | +----+-----------+-----------+ 6 rows in set (0.000 sec) So either insert a tag_check value too, or exclude tag_check from the UNIQUE key