Spring-data-jpa-duplicate-key-value-violates-unique-constraint

In a multi-threaded environment, two processes might check if a value (like an email address) exists at the same time. Both see that it doesn’t, both attempt to insert it, and the second one fails.

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 a multi-threaded environment, two processes might check

If you are manually assigning IDs to entities instead of using @GeneratedValue , you may inadvertently try to reuse an ID that is already present in the table. In a multi-threaded environment