entity cannot be tracked

背景：EF Core項目中使用InMemory做爲數據庫提供程序，編寫單元測試。

報錯：「The instance of entity type 'Movie' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached. Consider using 'DbContextOptionsBuilder.EnableSensitiveDataLogging' to see the conflicting key values.」

public class MovieServiceTests : TestBase
{
    private List<Movie> movies = new List<Movie>
    {
        new Movie{ Id = 1, ShortName = "復聯1" },
        new Movie{ Id = 2, ShortName = "復聯2" }
    };

    [Fact]
    public async Task DelMovieAsync_Test()
    {
        //Arrange
        dbContext.Movies.AddRange(movies);
        dbContext.SaveChanges();
        var entryState = dbContext.Entry(movies[0]).State;  // 此時爲Unchanged
        //Mark: movieService中的Remove方法和模擬數據(Arrange)時所用到的是同一個dbContext，此時movies對象的EntryState爲Unchanged
        //因爲實體對象還在被追蹤，致使The instance of entity type 'Movie' cannot be tracked
        dbContext.Movies.Attach(movies[0]).State = Microsoft.EntityFrameworkCore.EntityState.Detached;
        var movieService = new MovieService(mapper, dbContext, baseService.Object);
        //Act
        var movieId = movies[0].Id;
        var result = await movieService.DelMovieAsync(movieId);
        //Assert
        Assert.True(result.Code == CustomCodeEnum.Success);
    }
}

public async Task<Result> DelMovieAsync(int movieId)
{
    var result = new Result();
    _dbContext.Movies.Remove(new Movie { Id = movieId });
    var rows = await _dbContext.SaveChangesAsync();
    result.Content = rows > 0;
    return result;
}