In databases like PostgreSQL, the sequence used to generate IDs can sometimes fall behind the actual maximum ID in the table (often after manual data imports), leading the application to propose IDs that are already taken. Strategies for Resolution
Use a repository method like existsByEmail(String email) before attempting a save. While this doesn't solve high-concurrency race conditions, it eliminates the majority of "honest" mistakes. In databases like PostgreSQL, the sequence used to
Passing a detached entity to the save() method can sometimes lead JPA to treat it as a new record (attempting an INSERT ) rather than an update, causing a primary key collision. In databases like PostgreSQL