Entity Framework Core Insert entry with relationship 1 to many

Let me explain, i have 2 tables: Fuel and Vehicle. 1 Vehicle can have more Fuel entries, one Fuel entry has 1 vehicle. So it's a normal 1 - M relationship.

But when i try to insert a new Fuel and select a Vehicle that already exists this is the exception:

Duplicate entry '5' for key 'vehicle.PRIMARY'

It's like that when i try to create a new Fuel entry, instead linking the Fuel entry to an existing Vehicle, it try to create a new one. In fact, if i use postmap, and, instead of using an existing vehicle i put a new one, it create both Fuel and Vehicle entry.

This is the code i'm actually using: Fuel class:

public class Fuel
{
    [Key]
    public int id { get; set; }
    public DateTime date { get; set; }
    public float cost { get; set; }
    [ForeignKey("vehicleId")]
    public virtual Vehicle vehicle { get; set; }
    public int vehicleId { get; set; }
    public float startKm { get; set; }
    public float endKm { get; set; }
    public float liter { get; set; }
    public float average { get; set; }
}

Vehicle class:

 public class Vehicle
{
    [Key]
    public int id { get; set; }
    [Required, MaxLength(7), MinLength(7)]
    public string plate { get; set; }
    [JsonIgnore, InverseProperty("vehicle")]
    public ICollection<Fuel> Fuels { get; set; } = new HashSet<Fuel>();
}

DatabaseContext:

protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<JourneyAddress>()
            .HasKey(bc => new { bc.journeyId, bc.addressId });
        modelBuilder.Entity<JourneyAddress>()
            .HasOne(bc => bc.journey)
            .WithMany(b => b.journeyAddress)
            .HasForeignKey(bc => bc.journeyId)
            .OnDelete(DeleteBehavior.Cascade);
        modelBuilder.Entity<JourneyAddress>()
            .HasOne(bc => bc.address)
            .WithMany(c => c.JourneyAddress)
            .HasForeignKey(bc => bc.addressId)
            .OnDelete(DeleteBehavior.Cascade);
        modelBuilder.Entity<Fuel>()
            .HasOne(c => c.vehicle)
            .WithMany(e => e.Fuels)
            .HasForeignKey(c => c.vehicleId)
            .OnDelete(DeleteBehavior.Cascade);
    }

Controller:

public async Task<ActionResult<Fuel>> PostFuel(Fuel fuel)
    {
        _repo.Add(fuel);
        var save = await _repo.SaveAsync(fuel);

        return CreatedAtAction("GetFuel", new { id = fuel.id }, fuel);
    }

POSTMAP with an existing Vehicle
POSTMAP with a new Vehicle