Ensure that explicitly set primary key values are unique

I am getting following exception on my project:

An exception of type 'System.InvalidOperationException' occurred in EntityFramework.dll but was not handled in user code

Additional information: Saving or accepting changes failed because of more than one entity of type 'MyProject.Data.Poco.MyProjectCountry' have the same primary key value. Ensure that explicitly set primary key values are unique. Ensure that database-generated primary keys are configured correctly in the database and in the Entity Framework model. Use the Entity Designer for Database First/Model First configuration. Use the 'HasDatabaseGeneratedOption" fluent API or 'DatabaseGeneratedAttribute' for Code First configuration.

the error happens at the following line:

using (MyProjectDataContext context = new MyProjectDataContext())
{
    MyProjectItemTag existingItemTag = (from p in context.ItemTags.Include(p => p.MyProjectGenre).Include(p => p.MyProjectCountry)
                                     where p.MyProjectUser.UserId == ItemTag.MyProjectUser.UserId &&
                                           p.MyProjectItem.ItemId == MyProjectItem.ItemId
                                     select p).FirstOrDefault();


    // new tag
    if (existingItemTag == null)
    {
        existingItemTag = ItemTag;
        existingItemTag.MyProjectItem.ItemId = MyProjectItem.ItemId;
    }
    // existing tag
    else
    {
        existingItemTag.MyProjectItem = new MyProjectItem { ItemId = MyProjectItem.ItemId };
        existingItemTag.MyProjectUser = new MyProjectUser { UserId = ItemTag.MyProjectUser.UserId };
    }
    // updates
    existingItemTag.MyProjectCountry = MyProjectCountry;
    if (MyProjectCountry != null)
    existingItemTag.MyProjectCountry = new MyProjectCountry()
    {
        MyProjectCountryId = MyProjectCountry.MyProjectCountryId
    };
    existingItemTag.MyProjectGenre = MyProjectGenre;
    context.Entry(existingItemTag.MyProjectItem).State = EntityState.Unchanged;
    context.Entry(existingItemTag.MyProjectUser).State = EntityState.Unchanged;
    context.Entry(existingItemTag.MyProjectCountry).State = EntityState.Unchanged;
    context.Entry(existingItemTag.MyProjectGenre).State = EntityState.Unchanged;
    if (existingItemTag.MyProjectCountry != null)
    {
        context.Entry(existingItemTag.MyProjectCountry).State = EntityState.Unchanged;
    }
    // db
    context.ItemTags.AddOrUpdate(existingItemTag);
    context.SaveChanges();
    return existingItemTag.ItemTagId;
}

 context.Entry(existingItemTag.MyProjectCountry).State = EntityState.Unchanged;

My Class:

public class MyProjectItemTag
{
    public int ItemTagId { get; set; }
    public MyProjectUser MyProjectUser { get; set; }
    public MyProjectItem MyProjectItem { get; set; }
    public MyProjectCountry MyProjectCountry { get; set; }
    public MyProjectGenre MyProjectGenre { get; set; }
    public MyProjectMood MyProjectMood { get; set; }

    public MyProjectItemTag()
    {
    }

    public MyProjectItemTag(string userId, string providerContentId)
    {
        MyProjectUser = new MyProjectUser
        {
            UserId = userId
        };

        MyProjectItem = new MyProjectItem
        {
            ProviderContentId = providerContentId
        };
    }
}

My Config:

public class MyProjectItemTagConfiguration : EntityTypeConfiguration<MyProjectItemTag>
{
    public MyProjectItemTagConfiguration()
    {
        ToTable("MyProjectItemTags");
        HasKey(p => p.ItemTagId);
        HasRequired(p => p.MyProjectUser);
        HasRequired(p => p.MyProjectItem);
        HasOptional(p => p.MyProjectCountry);
    }
}

What I am missing here?