Error while testing PUT (with EntityState.Modified) request to a database using InMemory, xUnit in .NET unit tests

I do not have much experience with testing in .NET. I ran into an issue that I do not understand how to solve. Please advise where I can read about it.

I have a backend code that works with a simple database. I already created a frontend for it and everything works fine. But I wanted to add unit testing. I created a test project (VS template, xUnit) and added dependency Microsoft.EntityFrameworkCore.InMemory.

All my tests work, except a test for PUT. I receive the error: System.InvalidOperationException : The instance of entity type 'UserTask' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked. I tried to use .sqlite in memory instead of .inmemory and got the same error. My understanding is that I am supposed to test how _context.Entry(task).State = EntityState.Modified; await _context.SaveChangesAsync(); works and for some reason, I cannot test it because it doesn't detach the entity that I am replacing. ChatGPT suggested detaching it manually but then what am I testing there - it doesn't look to me like a valid test. Also, I found that I can use Context.Entry(entity).Reload(), but because I have a problem inside of the line var postResult = await _controller.PutTask(1,newTask), I don't think I can apply it.

Please direct me where I can find information about how to test this method.

In my controller in the main project:

    // PUT: api/Task/5
 [HttpPut("{id}")]
 public async Task<IActionResult> PutTask(int id, [FromBody] UserTask task)
 {
     if (id != task.Id ||
         !Enum.IsDefined(typeof(StatusEnum), task.Status) ||
         !Enum.IsDefined(typeof(PriorityEnum), task.Priority))
     { return BadRequest(); }

     _context.Entry(task).State = EntityState.Modified;

     try
     {
         await _context.SaveChangesAsync();
     }
     catch (DbUpdateConcurrencyException)
     {
         if (!TaskExists(id))
         {
             return NotFound();
         }
         else
         {
             throw;
         }
     }

     return NoContent();
 }

In my test file in the test project:

    public class TaskControllerTests : IDisposable
     {
         private readonly UserTasksContext _context;
         private readonly TaskController _controller;
    
         public TaskControllerTests()
         {
             var options = new DbContextOptionsBuilder<UserTasksContext>()
                 .UseInMemoryDatabase("TaskManagerTestDb")
                 .Options;
    
             _context = new UserTasksContext(options);
             _controller = new TaskController(_context);
    
             SeedDatabase();
         }
    
         private void SeedDatabase()
         {
             _context.UserTasks.AddRange(
                 new UserTask { Id = 1, Title = "Task 1", DueDate = DateTime.Now, Priority = "High", Status = "Pending" },
                 new UserTask { Id = 2, Title = "Task 2", DueDate = DateTime.Now, Priority = "Low", Status = "Completed" }
             );
             _context.SaveChanges();
         }
    
         public void Dispose()
         {
             _context.Database.EnsureDeleted();
             _context.Dispose();
         }
           [Fact]
  public async Task PutTask_EditsNew_ValidTask()
  {
      // Arrange
      var newTask = new UserTask {Id=1, Title = "New Task", DueDate = DateTime.Now.AddDays(3), Priority = "VeryHigh", Status = "InProgress" };

      // Act
      var postResult = await _controller.PutTask(1,newTask);
      var getResult = await _controller.GetTasks();

      // Assert
      var actionResult = Assert.IsType<ActionResult<IEnumerable<UserTask>>>(getResult);
      var tasks = Assert.IsAssignableFrom<IEnumerable<UserTask>>(actionResult.Value);
      Assert.Equal(2, tasks.Count());
      Assert.NotNull(tasks?.LastOrDefault());
      Assert.Equal("New Task", tasks?.FirstOrDefault()?.Title);
      Assert.Equal("InProgress", tasks?.FirstOrDefault()?.Status);
  }
}