외래키 중복 관련 질문

import com.example.banking.domain.user.User
import com.example.banking.support.BaseEntity
import jakarta.persistence.Entity
import jakarta.persistence.FetchType
import jakarta.persistence.JoinColumn
import jakarta.persistence.ManyToOne

@Entity
class Account(
    var balance: Double,
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    val user: User,

    id: Long = 0

) : BaseEntity(id) {
    fun addAccount(user: User) {
        user.accounts.add(this)
    }

    fun addBalance(amount: Double) {
        balance += amount
    }
    
    fun subtractBalance(amount: Double) {
        balance -= amount
    }
}

import com.example.banking.domain.account.Account
import com.example.banking.support.BaseEntity
import jakarta.persistence.*

@Entity
class Transaction(
    val amount: Double,
    @Enumerated(EnumType.STRING)
    val type: TransactionType,

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn
    val sender: Account,

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn
    val recipient: Account?,

    id: Long = 0L
) : BaseEntity(id) {
    constructor(amount: Double, type: TransactionType, sender: Account) : this(amount, type, sender, null)
}

import com.example.banking.domain.account.Account
import com.example.banking.domain.user.User
import com.example.banking.domain.user.UserInfo
import com.example.banking.repository.account.AccountRepository
import com.example.banking.repository.transaction.TransactionRepository
import com.example.banking.repository.user.UserRepository
import jakarta.persistence.EntityManager
import jakarta.transaction.Transactional
import org.junit.jupiter.api.Test
import org.springframework.beans.factory.annotation.Autowired
import org.springframework.boot.test.context.SpringBootTest
import org.springframework.test.annotation.Rollback

@SpringBootTest
@Transactional
@Rollback(value = false)
class TransactionTest(
    @Autowired private val em: EntityManager,
    @Autowired private val transactionRepository: TransactionRepository,
    @Autowired private val userRepository: UserRepository,
    @Autowired private val accountRepository: AccountRepository

) {
    @Test
    fun testTransaction() {
        val user1 = User(UserInfo("b", "Seoul", "123", "123"))
        val user2 = User(UserInfo("b2", "Seoul", "123", "123"))

        userRepository.save(user1)
        userRepository.save(user2)

        em.flush()
        em.clear()

        val account1 = Account(100.0, user1)
        val account2 = Account(100.0, user2)


        account1.addAccount(user1)
        account2.addAccount(user2)

        accountRepository.save(account1)
        accountRepository.save(account2)

        em.flush()
        em.clear()

        val sender = accountRepository.findById(account1.id).get()
        val recipient = accountRepository.findById(account2.id).get()


        val transaction = Transaction(50.0, TransactionType.TRANSFER, sender, recipient)
        transactionRepository.save(transaction)

        em.flush()

        em.clear()


        val transaction2 = Transaction(50.0, TransactionType.DEPOSIT, sender)

        transactionRepository.save(transaction2)


//        assertThat(sender.balance).isEqualTo(50.0)
//        assertThat(recipient.balance).isEqualTo(150.0)

    }
}

위와같이 Transaction 엔티티와 Account 엔티티가 ManyToOne 으로 매핑되었을때, 테스트코드를 위와같이 작성시,

transactionRepository.save(transaction2)

이 라인에서 Duplicate entry '97' for key 'transaction.UK_96vb4d846be64bta5qbxiicb1' 에러가 발생합니다.
제가 알기론 외래키의 경우 중복된 값을 가지는것이 가능한것으로 알고 있는데, 같은 account id로 새로운 Transaction 을 생성할 시에 키가 중복이라는 에러가 발생하는데 왜 그런지 알고싶습니다.

코틀린으로 코드 작성한 점 양해부탁드립니다..