EF Core tries to add new record instead of update 1:many relation

I'm not sure why while trying to create an entity which is 1:many EF tries to add new entry in Asp Net Users instead of update 1:many

I have one user which has many items

SqlException: Violation of PRIMARY KEY constraint 'PK_AspNetUsers'. Cannot insert duplicate key in object 'dbo.AspNetUsers'. The duplicate key value is (cdbb1f2f-ddcf-40c0-97ec-f50f8049d87a).

public class Context : IdentityDbContext
{
    public Context(DbContextOptions options) : base(options)
    {
    }

    public DbSet<User> Users { get; set; }

    public DbSet<Item> Items { get; set; }

    public DbSet<File> Files { get; set; }
}

public class User : IdentityUser
{
    public List<Item> Items { get; set; } = new List<Item>();
}

public class Item
{
    private Item()
    {

    }

    public Item(string title, User owner, File file)
    {
        Title = title;
        Owner = owner;
        File = file;
    }

    public int Id { get; private set; }

    public string Title { get; set; }

    public User Owner { get; set; }

    public File File { get; set; }

    public DateTime CreationDate { get; } = DateTime.Now;
}

And here's where's the problem:

var fileResult = await _file.SaveFile(input.File);
var item = new Item(input.Title, user, fileResult.File);

user.Items.Add(item);
await _context.Items.AddAsync(item);
await _context.SaveChangesAsync();

User is loaded with:

public User GetUser()
{
    return _context.Users.FirstOrDefault(x => x.UserName == _http.HttpContext.User.Identity.Name);
}

I tried that:

When I change

public Item(string title, User owner, File file)
{
    Title = title;
    Owner = owner;
    File = file;
}

to just:

public Item(string title, File file)
{
    Title = title;
    File = file;
}

and let it be handled by:

user.Items.Add(item);

then OwnerId in DB is null

Using:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    modelBuilder.Entity<User>()
        .HasMany(x => x.Items)
        .WithOne(x => x.Owner);

    modelBuilder.Entity<Item>().HasOne(x => x.Owner);
}

is not helping either