How to avoid duplicate insert in Entity Framework 4.3.1

I have a small model created using code-first approach - a class City which contains only information about city name.

public class City
{
    public City()
    {
        Posts = new List<Post>();
    }

    public City(string cityName)
    {
        Name = cityName;
    }

    public virtual ICollection<Post> Posts { get; private set; }
    public int Id { get; set; }
    public string Name { get; private set; }
}

A Post class represents combination of zip code and city reference

public class Post
{
    public virtual City City { get; set; }        
    public int Id { get; set; }
    public string ZipCode { get; set; }        
}

both entities have their sets defined in context as their configurations

public DbSet<City> Cities { get; set; }
public DbSet<Post> Posts { get; set; }

modelBuilder.Configurations.Add(new CityMap());
modelBuilder.Configurations.Add(new PostMap());


public class CityMap : EntityTypeConfiguration<City>
{
    public CityMap()
    {
        // Primary Key
        HasKey(t => t.Id);

        // Properties
        // Table & Column Mappings
        ToTable("City");
        Property(t => t.Id).HasColumnName("Id");
        Property(t => t.Name).HasColumnName("Name");
    }
}

public class PostMap : EntityTypeConfiguration<Post>
{
    public PostMap()
    {
        // Primary Key
        HasKey(t => t.Id);

        // Properties
        // Table & Column Mappings
        ToTable("Post");
        Property(t => t.Id).HasColumnName("Id");            
        Property(t => t.ZipCode).HasColumnName("ZipCode");

        // Relationships
        HasRequired(t => t.City)
        .WithMany(t => t.Posts)
        .Map(map=>map.MapKey("CityId"));
    }
}

I've created class for manipulation with those objects with static methods which get or creates objects and return them to caller.

private static City GetCity(string cityName)
{
        City city;

        using (var db = new DbContext())
        {
            city = db.Cities.SingleOrDefault(c => c.Name == cityName);

            if (city == null)
            {
                city = new City(cityName);
                db.Cities.Add(city);
                db.SaveChanges();                    
            }
        }

        return city;
    }

    private static Post GetPost(string zipCode, string cityName)
    {
        Post post;

        City city = GetCity(cityName);

        using (var db = new DbContext())
        {      
            post = db.Posts.SingleOrDefault(p => p.City.Id == city.Id && p.ZipCode == zipCode);
            if (post == null)
            {
                post = new Post { City = city, ZipCode = zipCode };

                // State of city is unchanged
                db.Posts.Add(post);

                // State of city is Added
                db.SaveChanges();
            }
        }

        return post;
    }

Imagine, that I call method

GetPost("11000","Prague");

method GetCity is started and if not exists, method creates a city and then calls the SaveChanges() method.

If I set returned city entity to new Post instance, Entity Framework generates a second insert for the same city.

How can I avoid this behavior? I want to only insert new post entity with referenced city created or loaded in previous step.

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