Error when trying to use EF Functions with Generic IRepository Pattern

i have implemented the Generic Repository Pattern in my app , with unity container as DI provider but when i start testing if it works or not this error appear " An entity object cannot be referenced by multiple instances of IEntityChangeTracker "

my Repository Code

public interface IRepository<T> where T : class
    {
        IEnumerable<T> GetAll();
        T Get(int id);
        T Add(T item);
        bool Update(T item);
        bool Delete(T item);
        IEnumerable<T> Find(Func<T, bool> predicate);
        void Save();
    }

    public class Repository<M, T> : IRepository<T> where T : class where M : DbContext, new()
    {
        private ILogger _Log;

        public Repository(ILogger log)
        {
            _Log = log;
        }

        private M _entities = new M();
        public M Context
        {

            get { return _entities; }
            set { _entities = value; }
        }

        public T Add(T item)
        {
            try
            {
                _entities.Set<T>().Add(item);
                return null;
            }
            catch (Exception ex)
            {
                _Log.LogErorr(this.GetType().Name + "Add Method", ex.Message + "--" + ex.InnerException);
                throw;
            }
        }

        public virtual bool Delete(T item)
        {
            try
            {
                _entities.Set<T>().Remove(item);
                return true;
            }
            catch (Exception ex)
            {
                _Log.LogErorr(this.GetType().Name + "Delete Method", ex.Message + "--" + ex.InnerException);
                throw;
            }

        }

        public IEnumerable<T> Find(Func<T, bool> predicate)
        {
            IEnumerable<T> query = _entities.Set<T>().Where(predicate);
            return query;
        }

        public T Get(int id)
        {
            return _entities.Set<T>().FirstOrDefault();
        }

        public IEnumerable<T> GetAll()
        {
            IEnumerable<T> data = _entities.Set<T>();
            return data;
        }

        public bool Update(T item)
        {
            try
            {
                _entities.Entry(item).State = EntityState.Modified;
                return true;
            }
            catch (Exception ex)
            {

                _Log.LogErorr(this.GetType().Name + "Update Method", ex.Message + "--" + ex.InnerException);
                throw;
            }
        }

        public virtual void Save()
        {
            try
            {
                _entities.SaveChanges();
            }
            catch (Exception ex)
            {
                _Log.LogErorr(this.GetType().Name + "Save Method", ex.Message + "--" + ex.InnerException);
                throw;
            }
        }
    }
}

then i try to add some dummy data to the DB by this code and inject the controller with service

public class HomeController : Controller
    {
 private IRepository<account> _AccountRepository;
        public HomeController(ILogger Log,
                              IRepository<account> AccountRepository)
        {
            _Log = Log;
            _AccountRepository = AccountRepository;
        }
account ac = new account()
 _AccountRepository.Add(ac);
_AccountRepository.Save();

}