How to make a Java function that checks if data from a POST request is already in the database?

Question

So I'm making an API using Java Spring-boot and JBDC with PostgreSQL as the database and Postman for testing. All the basic CRUD function is already implemented, but I'm trying to validates if a value in the POST request is already in the database or not so there are no duplicate data.

Here's the model:

package com.akuntansi.akuntansi.model;

import java.sql.Timestamp;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

public class mAkuntansi {
    private Integer id_md_jurnal_Integer;
    private Integer id_dd_wilayah_kerja;
    private String kode_file;
    private String file_id;
    private String tanggal;
    private String debet_kredit;
    private String kode_coa;
    private long nominal;
    private Integer flag;
    private Integer id_akmt_subledger;
    private Integer id_referensi;
    private String no_bukti;
    private Timestamp created_date;

    public mAkuntansi(){}

    public mAkuntansi(Integer id_md_jurnal_Integer, Integer id_dd_wilayah_kerja, String kode_file, String file_id, String tanggal, String debet_kredit, String kode_coa, 
    long nominal, Integer flag, Integer id_akmt_subledger, Integer id_referensi, String no_bukti, Timestamp created_date){
        this.id_md_jurnal_Integer = id_md_jurnal_Integer;
        this.id_dd_wilayah_kerja = id_dd_wilayah_kerja;
        this.kode_file = kode_file;
        this.file_id = file_id;
        this.tanggal = tanggal;
        this.debet_kredit = debet_kredit;
        this.kode_coa = kode_coa;
        this.nominal = nominal;
        this.flag = flag;
        this.id_akmt_subledger = id_akmt_subledger;
        this.id_referensi = id_referensi;
        this.no_bukti = no_bukti;
        this.created_date = created_date;
    }

    public mAkuntansi(Integer id_dd_wilayah_kerja, String kode_file, String file_id, String tanggal, String debet_kredit, String kode_coa, 
    long nominal, Integer flag, Integer id_akmt_subledger, Integer id_referensi, String no_bukti, Timestamp created_date){
        this.id_dd_wilayah_kerja = id_dd_wilayah_kerja;
        this.kode_file = kode_file;
        this.file_id = file_id;
        this.tanggal = tanggal;
        this.debet_kredit = debet_kredit;
        this.kode_coa = kode_coa;
        this.nominal = nominal;
        this.flag = flag;
        this.id_akmt_subledger = id_akmt_subledger;
        this.id_referensi = id_referensi;
        this.no_bukti = no_bukti;
        this.created_date = created_date;
    }
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    public Integer getid_md_jurnal_int(){
        return id_md_jurnal_Integer;
    }

    public void setid_md_jurnal_int(Integer id_md_jurnal_Integer){
        this.id_md_jurnal_Integer = id_md_jurnal_Integer;
    }

    public Integer getid_dd_wilayah_kerja(){
        return id_dd_wilayah_kerja;
    }

    public void setid_dd_wilayah_kerja(Integer id_dd_wilayah_kerja){
        this.id_dd_wilayah_kerja = id_dd_wilayah_kerja;
    }

    public String getkode_file(){
        return kode_file;
    }

    public void setkode_file(String kode_file){
        this.kode_file = kode_file;
    }

    public String getfile_id(){
        return file_id;
    }

    public void setfile_id(String file_id){
        this.file_id = file_id;
    }

    public String gettanggal(){
        return tanggal;
    }

    public void settanggal(String tanggal){
        this.tanggal = tanggal;
    }

    public String getdebet_kredit(){
        return debet_kredit;
    }

    public void setdebet_kredit(String debet_kredit){
        this.debet_kredit = debet_kredit;
    }

    public String getkode_coa(){
        return kode_coa;
    }

    public void setkode_coa(String kode_coa){
        this.kode_coa = kode_coa;
    }

    public long getnominal(){
        return nominal;
    }

    public void setnominal(long nominal){
        this.nominal = nominal;
    }

    public Integer getflag(){
        return flag;
    }

    public void setflag(Integer flag){
        this.flag = flag;
    }

    public Integer getid_akmt_subledger(){
        return id_akmt_subledger;
    }

    public void setid_akmt_subledger(Integer id_akmt_subledger){
        this.id_akmt_subledger = id_akmt_subledger;
    }

    public Integer getid_referensi(){
        return id_referensi;
    }

    public void setid_referensi(Integer id_refrensi){
        this.id_referensi = id_refrensi;
    }

    public String getno_bukti(){
        return no_bukti;
    }

    public void setno_bukti(String no_bukti){
        this.no_bukti = no_bukti;
    }

    public Timestamp getcreated_date(){
        return created_date;
    }

    public void setcreated_date(Timestamp created_date){
        this.created_date = created_date;
    }

}

Let say for example I want to validates the kode_coa. There's a POST request with kode_coa of K1 but in the database, a data with value K1 already exist, how do you prevent a duplicate data from getting inserted into the database?