When setting up Crowd using MySQL (version 5.6.24) and then syncing Crowd 2.8.2 to a remote Crowd instance (e.g. Extranet Crowd), we are seeing com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '131399-66649-GROUP_USER' for key 'uk_mem_parent_child_type' errors when syncing for the second time (and thereafter).
We are not seeing these errors when using a PostgreSQL database.
Most likely these errors can also happen with certain versions of HSQLDB. We experienced them in Stash (see issue https://sdog.jira.com/browse/LEM-726) which is using version 2.2.4. Crowd 2.8.2 is using an older version (126.96.36.199) and that might be the reason that the error can only be reproduced with mySQL.