Doctrine saves entities twice - symfony

Question

Iam using Symfony(4.3) and Doctrine. When I want to get a user like this in the constructor of the script:
$userRepo = $this->em->getRepository(User::class);
$this->systemUser = $userRepo->findOneBy([
"firstName" => "system",
"lastName" => "system",
]);
And save the reference to this user for example at a createdBy-field like this:
$newUser->setName("test");
$newUser->setCreatedBy($this->systemUser);
$this->entityManager->persist($newUser);
$this->entityManager->flush();
The systemUser is persisted to the database after every flush().
I already tried to get only the reference with
$this->systemUser = $this->entityManager->getReference(User::class, $this->systemUser->getId());
But this doesn't work either.
Edit:
The setCreatedBy-Method:
public function setCreatedBy(User $user): void {
$this->createdBy = $user;
}
The 'createdBy'-Field:
/**
* #var User
* #ORM\ManyToOne(targetEntity="App\Entity\User", cascade={"persist"})
* #ORM\JoinColumn(nullable=true)
*/
protected $createdBy;

Answer 1

You have configured the cascade={"persist"} option for the relation which means Doctrine will save "new" entities found through the relation.
As your question suggests the createdBy field can only contain a relationship to already existing users. This means cascade is definitely not necessary here. It's also the reason why existing entities are persisted again in your case.
Remove the cascade option from the mapping-configuration for the property to resolve your issue.
Clear your cache afterwards.

Answer 2

To answer my own question:
I forgot to mention that I insert ~ 70.000 items in a loop and use clear() every 100 rows. The problem is the use of the entity manager's clear()-method. it detaches all doctrine-managed entites so doctrine thinks that these entities are new and saves them to the db. So I have to do it like this:
$newUser->setCreatedBy($this->em->getReference(User::class, $this->systemUser->getId()));