Sequence contains more than one element error in MVC 5

Question

I created some models, added the migration and then did an update database operation, though at my last update database operation I got the error message saying:
    Sequence contains more than one element
Below you can find my migration configuration:
        context.Categories.AddOrUpdate(p => p.CategoryName,
            new Category
                CategoryName = "Sport"
            new Category
                CategoryName = "Music"
        context.Subcategories.AddOrUpdate(p => p.SubcategoryName,
            new Subcategory
                SubcategoryName = "Football"
            new Subcategory
                SubcategoryName = "Basketball"
            new Subcategory
                SubcategoryName = "Piano"
            new Subcategory
                SubcategoryName = "Violin"
        context.Services.AddOrUpdate(p => p.ServiceType,
            new Service
                ServiceType = "Football player",
                Category = { CategoryName = "Sport" },
                Subcategory = { SubcategoryName = "Football" }
            new Service 
                ServiceType = "Piano lessons",
                Category = { CategoryName = "Music" },
                Subcategory = { SubcategoryName = "Piano" }
The problem occurs with when I add new Services. I already have categories and subcategories, and if I do like Category = new Category { CategoryName = "Music" } then it works but I get Music entry twice in my database (for this example). I want to
use the already added categories and subcategories. Below also you can find my models definitions.
public class Category
    [Key]
    public int CategoryID { get; set; }
    public string CategoryName { get; set; }
// Subcategory is defined the same way...
public class Service
    public int ServiceID { get; set; }
    public string ServiceType { get; set; }
    public virtual Category Category { get; set; }
    public virtual Subcategory Subcategory { get; set; }
}

Answer 1

After reading the article in the link that you have provided, I did the following changes in my models, and created controllers for each of them using Entity Framework, then I created a migration and named it InitialServices. Afterwards, I added a few
entries in my Configuration.cs file and when I typed Update-Database, I got an error message in package manager saying "RenameIndexOperation", which is marked with red. Below you can find my changed models and my Configuration.cs file, along with the migration
file created automatically.
Category.cs:
    public class Category
        [Key]
        public int CategoryID { get; set; }
        public string CategoryName { get; set; }
        public virtual ICollection<Subcategory> Subcategories { get; set; }
Subcategory.cs:
    public class Subcategory
        [Key]
        public int SubcategoryID { get; set; }
        public string SubcategoryName { get; set; }
        [ForeignKey("Category")]
        public int CategoryID { get; set; }
        public virtual Category Category { get; set; }
        public virtual ICollection<Service> Services { get; set; }
Service.cs:
    public class Service
        [Key]
        public int ServiceID { get; set; }
        [Required]
        [Display(Name="Service type")]
        public string ServiceType { get; set; }
        [ForeignKey("Subcategory")]
        public int SubcategoryID { get; set; }
        public int Count { get; set; }
        public virtual Subcategory Subcategory { get; set; }
_InitialServices.cs:
    public partial class InitialServices : DbMigration
        public override void Up()
            DropForeignKey("dbo.Services", "Category_CategoryID", "dbo.Categories");
            DropIndex("dbo.Services", new[] { "Category_CategoryID" });
            RenameColumn(table: "dbo.Services", name: "Subcategory_SubcategoryID", newName: "SubcategoryID");
            RenameIndex(table: "dbo.Services", name: "IX_Subcategory_SubcategoryID", newName: "IX_SubcategoryID");
            AddColumn("dbo.Subcategories", "CategoryID", c => c.Int(nullable: false));
            CreateIndex("dbo.Subcategories", "CategoryID");
            AddForeignKey("dbo.Subcategories", "CategoryID", "dbo.Categories", "CategoryID", cascadeDelete: true);
            DropColumn("dbo.Services", "Category_CategoryID");
        public override void Down()
            AddColumn("dbo.Services", "Category_CategoryID", c => c.Int(nullable: false));
            DropForeignKey("dbo.Subcategories", "CategoryID", "dbo.Categories");
            DropIndex("dbo.Subcategories", new[] { "CategoryID" });
            DropColumn("dbo.Subcategories", "CategoryID");
            RenameIndex(table: "dbo.Services", name: "IX_SubcategoryID", newName: "IX_Subcategory_SubcategoryID");
            RenameColumn(table: "dbo.Services", name: "SubcategoryID", newName: "Subcategory_SubcategoryID");
            CreateIndex("dbo.Services", "Category_CategoryID");
            AddForeignKey("dbo.Services", "Category_CategoryID", "dbo.Categories", "CategoryID", cascadeDelete: true);
Configuration.cs:
protected override void Seed(Workfly.Models.ApplicationDbContext context)
            var categories = new List<Category>
                new Category { CategoryName = "Sport" },
                new Category { CategoryName = "Music" }
            categories.ForEach(c => context.Categories.AddOrUpdate(p => p.CategoryName, c));
            context.SaveChanges();
            var subcategories = new List<Subcategory>
                new Subcategory { SubcategoryName = "Football", CategoryID = categories.Single(c => c.CategoryName == "Sport").CategoryID },
                new Subcategory { SubcategoryName = "Basketball", CategoryID = categories.Single(c => c.CategoryName == "Sport").CategoryID },
                new Subcategory { SubcategoryName = "Piano", CategoryID = categories.Single(c => c.CategoryName == "Music").CategoryID },
                new Subcategory { SubcategoryName = "Violin", CategoryID = categories.Single(c => c.CategoryName == "Music").CategoryID }
            foreach (Subcategory s in subcategories)
                var subcategoriesInDB = context.Subcategories.Where(c => c.Category.CategoryID == s.CategoryID).SingleOrDefault();
                if (subcategoriesInDB == null)
                    context.Subcategories.Add(s);
            context.SaveChanges();
            var services = new List<Service>
                new Service { ServiceType = "Football coach", SubcategoryID = subcategories.Single(s => s.SubcategoryName == "Football").SubcategoryID },
                new Service { ServiceType = "Piano lessons", SubcategoryID = subcategories.Single(s => s.SubcategoryName == "Music").SubcategoryID }
            foreach (Service s in services)
                var servicesInDB = context.Services.Where(t => t.Subcategory.SubcategoryID == s.SubcategoryID).SingleOrDefault();
                if (servicesInDB == null)
                    context.Services.Add(s);
            context.SaveChanges();
        }