In Nina QuickStart, an SQL file is executed during the import process, inserting a duplicate entry into #__schemas, thus creating an existing entry. This causes the import to fail with a duplicate key error.
The affected file in the QuickStart package is:
installation/sql/mysql/custom.sql
It contains (around line 1895 in my case) the following statement:
INSERT INTO `#__schemas` VALUES (21,'5.0.0-2023-09-11'),(700,'5.3.0-2025-03-14'),(1254,'3.5.1');
The part
(700,'5.3.0-2025-03-14') is the culprit.
Solution: In the file installation/sql/mysql/custom.sql, remove the entry for 700:
Before:
INSERT INTO `#__schemas` VALUES (21,'5.0.0-2023-09-11'),(700,'5.3.0-2025-03-14'),(1254,'3.5.1');
After:
INSERT INTO `#__schemas` VALUES (21,'5.0.0-2023-09-11'),(1254,'3.5.1');
Then reinstall QuickStart (ideally with a completely empty/new database), and the installation should complete successfully.
Best regards