ASP MVC Entity Framework core. Update many to many relationship on form post

Question

I'd like to know a good practice to update a many to many relationship when submit form.

I got these two entities and I use the default many to many relationship from EF core 5:

public class BlogEntry
   {
      public int Id { get; set; }
      [Required]
      [MaxLength(200)]
      public string Title { get; set; }
      [Required]
      public string EntryText { get; set; }
      [NotMapped] 
      public IEnumerable<string> CategoriesToPublish { get; set; } 
      public ICollection<Category> Categories { get; set; } 

   }

   public class Category
   {
      public int Id { get; set; }
      public string Name { get; set; }
      public ICollection<BlogEntry> BlogEntries { get; set; }
   }

context:

      public DbSet<BlogEntry> BlogEntries { get; set; }
      public DbSet<Category> Categories { get; set; }

And I have a form witha multiselect field to represent this relationship. See Image form

I'm not using the relation property on the form(maube I should, but I don't know), I have another property to convert the relationship into a list of strings called CategoriesToPublish so I can load the multiselect and retrieve the selection on post.

On the post action method, I want to iterate the this CategoriesToPublish and update all the relationships.

      [HttpPost]
      [ValidateAntiForgeryToken]
      public async Task<IActionResult> Publish(BlogEntry blogEntry)
      {
         if (ModelState.IsValid)
         {
            blogEntry.Categories = await _context.Categories.Where(x => x.BlogEntries.Any(x => x.Id == blogEntry.Id)).ToListAsync();
            await UpdateCategories(blogEntry);
            _context.Update(blogEntry);
            await _context.SaveChangesAsync();
            return RedirectToAction(nameof(List));
         }
         return View(blogEntry);
      }

But the problem that I'm facing is that the Categories relationship is not loaded on postback. And if I try to load it manually and save context, I get an error saying SqlException: Violation of PRIMARY KEY constraint 'PK_BlogEntryCategory'. Cannot insert duplicate key in object 'dbo.BlogEntryCategory'

I am not sure how to approach this problem. Any advice?