While trying to add rows to a table using Hibernate, NonUniqueObjectException is thrown. Why?

Computer Science
1

Earlier, I successfully added an annotated class instance to a database table. At some point, I truncated it (at this point, it’s empty), and now when I try to add another row (or two), I get this exception

17:55:07.405 [main] ERROR org.hibernate.orm.connections.pooling - Connection leak detected: there are 1 unclosed connections upon shutting down pool jdbc:mysql://localhost:3306/my_db?useSSL=false&serverTimezone=UTC
17:55:07.418 [main] ERROR org.hibernate.orm.connections.pooling - Connection leak detected: there are 1 unclosed connections
Exception in thread "main" org.hibernate.NonUniqueObjectException: A different object with the same identifier value was already associated with the session : [org.example.Employee#0]
	at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:179)
	at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:134)
	at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:197)
	at org.hibernate.event.internal.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
	at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:182)
	at org.hibernate.event.internal.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:28)
	at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:78)
	at org.hibernate.event.service.internal.EventListenerGroupImpl.fireEventOnEachListener(EventListenerGroupImpl.java:107)
	at org.hibernate.internal.SessionImpl.fireSave(SessionImpl.java:649)
	at org.hibernate.internal.SessionImpl.save(SessionImpl.java:641)
	at org.hibernate.internal.SessionImpl.save(SessionImpl.java:636)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.hibernate.context.internal.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:345)
	at jdk.proxy2/jdk.proxy2.$Proxy50.save(Unknown Source)
	at org.example.Test.main(Test.java:20)

Why is it happening, and how do I solve this problem?

Here’s the repo if you need to check out the entire code