The instance of entity type 'XY' cannot be tracked because another instance with the same key value for {'XId', 'YId'} is already being tracked

Question

Sorry to raise this one again - I see it plenty of times on here but I am still puzzled about it in my case, especially because I seem to be able to 'overcome' it, but I'm not sure I like how, and I would love to understand why.

I have a many to many relationship between Staff and Departments, with the StaffDepartment table being set up with a composite key like so:

modelBuilder.Entity<StaffDepartment>() .HasKey(sd => new { sd.StaffId, sd.DepartmentId });

I suspect that must be something to do with the exception I sometimes get:

InvalidOperationException: The instance of entity type 'StaffDepartment' cannot be tracked because another instance with the same key value for {'StaffId', 'DepartmentId'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached.

I say sometimes because, strangely, if I change the department in my staff details page and update, it goes through fine, but when I leave it the same (maybe just changing the staff name for example) I receive this exception.

I tried implementing the various suggestions on this similar but apparently different SO discussion and all sorts of ways of tinkering with my repository update method, and strangely enough this is the only way I have got it working:

public async Task UpdateStaff(StaffDto staff)
        {
            var staffToUpdate = await _db.Staff
                .Include(d => d.Departments)
                .ThenInclude(d => d.Department)
                .SingleOrDefaultAsync(s => s.Id == staff.Id);

            // Without either these two line I get the exception
              _db.StaffDepartment.RemoveRange(staffToUpdate.Departments);
              await _db.SaveChangesAsync();               

            _mapper.Map(staff, staffToUpdate);
            await _db.SaveChangesAsync();
        }

So it seems strange that I should have to call SaveChangesAsync() twice in same method. What is actually happening there? And how would I achieve the suggestion in the error When attaching existing entities, ensure that only one entity instance with a given key value is attached without doing so.

Entity classes look like so:

public class Staff
    {
        public int Id { get; set; }
        ...
        public ICollection<StaffDepartment> Departments { get; set; }
    }

public class Department
    {
        public int DepartmentId { get; set; }
        ...
        public ICollection<StaffDepartment> Staff { get; set; }
    }

public class StaffDepartment
    {
        public int StaffId { get; set; }
        public Staff Staff { get; set; }
        public int DepartmentId { get; set; }
        public Department Department { get; set; }

    }

And the staffDto that comes back from the domain etc. is simply:

public class StaffDto
    {
        public int Id { get; set; }
        ...
        public List<DepartmentDto> Departments { get; set; }
    }

The repositories are all registered as transient i.e.

services.AddTransient<ICPDRepository, CPDRepository>();