Duplicate key exception when adding a user to a group - keycloak

Question

I've written an EventListenerProvider designed to automatically add a new user to a group after the user completes registration. The EventListenerProvider looks for the REGISTER event and invokes some code that gets the UserModel:
UserModel user = session.getContext().getAuthenticationSession().getAuthenticatedUser();
the group is created if it doesn't exist:
group = session.getContext().getRealm().createGroup(groupName);
and then the user is added to the group:
user.joinGroup(group);
The problem is that for some reason, joinGroup results in an INSERT on the user_entity table using the same id as the user that was supposedly created during registration. This results in a unique key violation with the following partial stack trace:
keycloak_1 | 22:44:17,992 WARN [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (default task-1) SQL Error: 0, SQLState: 23505
keycloak_1 | 22:44:17,992 ERROR [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (default task-1) ERROR: duplicate key value violates unique constraint "constraint_fb"
keycloak_1 | Detail: Key (id)=(3e707ca2-6311-4d0b-bd56-786746b172f0) already exists.
keycloak_1 | 22:44:17,993 ERROR [org.keycloak.events.EventBuilder] (default task-1) Failed to send type to com.abinitio.sso.keycloak.event.EventListener#4d6c5c44: org.keycloak.models.ModelDuplicateException: javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement
keycloak_1 | at org.keycloak.keycloak-model-jpa#10.0.1//org.keycloak.connections.jpa.PersistenceExceptionConverter.convert(PersistenceExceptionConverter.java:57)
keycloak_1 | at org.keycloak.keycloak-model-jpa#10.0.1//org.keycloak.connections.jpa.PersistenceExceptionConverter.invoke(PersistenceExceptionConverter.java:51)
keycloak_1 | at javax.persistence.api#2.2.3//com.sun.proxy.$Proxy95.flush(Unknown Source)
keycloak_1 | at org.keycloak.keycloak-model-jpa#10.0.1//org.keycloak.models.jpa.UserAdapter.joinGroupImpl(UserAdapter.java:389)
The Postgres log shows the following:
postgres_1 | 2020-05-20 22:44:17.991 UTC [34] ERROR: duplicate key value violates unique constraint "constraint_fb"
postgres_1 | 2020-05-20 22:44:17.991 UTC [34] DETAIL: Key (id)=(3e707ca2-6311-4d0b-bd56-786746b172f0) already exists.
postgres_1 | 2020-05-20 22:44:17.991 UTC [34] STATEMENT: insert into public.USER_ENTITY (CREATED_TIMESTAMP, EMAIL, EMAIL_CONSTRAINT, EMAIL_VERIFIED, ENABLED, FEDERATION_LINK, FIRST_NAME, LAST_NAME, NOT_BEFORE, REALM_ID, SERVICE_ACCOUNT_CLIENT_LINK, USERNAME, ID) values ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13)
I'm not quite sure why the joinGroup call results in an INSERT on the user_entity table.
After this error, the user that was registering is not in the DB. So I'm assuming the user create resulting from registration was never committed. Perhaps this was still pending which is why the INSERT on the user_entity table happens when joinGroup tried to commit? I tried calling EntityManager.flush() just prior to call joinGroup(), but this didn't solve the problem.
Hopeful for some suggestions...