How update entity after adding new entity in EF Core

I have a two tables Client and ClientSecret. The tables have following structure:

public class Client
    {
        public int Id { get; set; }
        public string ClientId { get; set; }
        public string ClientName { get; set; }
    public List<ClientSecret> ClientSecrets { get; set; }
    }

public class ClientSecret
{
        public int Id { get; set; }
        public string Description { get; set; }
        public string Value { get; set; }
        public DateTime? Expiration { get; set; }
        public Client Client { get; set; }
}

I need for of all create a new Client set the ClientId and ClientName and get back a new Id of this object from DB.

I have tried this (_dbContext is getting via DI system in the constructor):

var newClient = new Client();
newClient.ClientName = client.ClientName;
newClient.ClientId = client.ClientId;
_dbContext.Clients.Add(newClient);
await _dbContext.SaveChangesAsync();

var newClientId = newClient.Id;
var clientToUpdate = await _dbContext.Clients.Where(x => x.Id == newClientId).SingleOrDefaultAsync();

clientToUpdate.ClientSecrets.AddRange(secrets);
_dbContext.Clients.Update(clientToUpdate);
await _dbContext.SaveChangesAsync();

I need first create a Client because the table ClientSecrets needs a ClientId for creating a new records into this table.

I got this error message:

Failed the in the row with: _dbContext.Clients.Update(clientToUpdate);

InvalidOperationException: The instance of entity type 'Client' cannot be tracked because another instance of this type with the same key is already being tracked. When adding new entities, for most key types a unique temporary key value will be created if no key is set (i.e. if the key property is assigned the default value for its type). If you are explicitly setting key values for new entities, ensure they do not collide with existing entities or temporary values generated for other new entities. When attaching existing entities, ensure that only one entity instance with a given key value is attached to the context.

How can I fix this issue?