EFCore Collection Trying to save a duplicate.

    public class Rental
    {
        public int Id {  get; set; }
        public int CustomerId { get; set; }
        public Customer Customer { get; set; }
        public ICollection<Equipment> Equipment { get; } = new List<Equipment>();
        public DateTime RentalDate { get; set; }
        public DateTime ReturnDate {  get; set; }
        [JsonIgnore]
        public float TotalCost => Equipment.Sum(p => p.Cost);
    }

    public class Equipment
    {
        public int Id { get; set; }
        public int CategoryId { get; set; }
        public Category Category { get; set; }
        [JsonIgnore]
        public string CategoryName => Category.Name;
        public string Name { get; set; }
        public string Description { get; set; }
        public float Cost { get; set; }
    }

        Rental createdRental = (Rental)result.Data;
        if (createdRental == null)
        {
            return;
        }
        db.Update(createdRental);
        await db.SaveChangesAsync();
        StateHasChanged();