Entity Framework inserting duplicates on Seeding d

This question already has an answer here:

Entity Framework database mapping relationships (Duplicate creation using Seed() method) (1 answer)

EDIT----

From here i tried assigning Id's in the seeding method and this was OK for Languages but not when i added Address to the customer and assign Id's as well to these addresses, than it created the dupes again...

Both Address & Language are declared as DbSet<...> in my Context

What i tried:

Adding 1 Address (with Id) - add this to 1 customer => Creates a dupe
Adding 1 language & 1 Address (with Id's) - add both to 1 customer => Creates a dupe
Adding 1 Customer with nothing buts its name => Doesn't create a dupe
Adding 1 language (with Id) - add this to 1 customer => Doesn't create a dupe

I have a Override ToString() method on my Customer which return its name, I can observe that when i look at the duplicate while debugging 1 is with the name, the other is the Namespace where the Customer class resides in, which seems loguc since the Name is NULL in the Dupes case but i figured to mention it anyway ...

----EDIT

I am seeding my database with some metadata and i see that it has a very strange behavior i never saw before. I am inserting a Entity "Customer" and it inserts this entity 2 times, first insert is correct and has everything it should have, the other one has NULL properties (string values) but some (like datetimes) have values.

I have totally no clue why this is happening, it is occurring when i call the base.Seed(ctx); method, that i am sure since i stopped the Webapp after this before it reached anything else.

This entity Customer has a related Entity Language as well as a Collection of Addresses.

I have another post open (no suggestions yet) where the same issue occurs and this happened suddenly, i did not make any changes myself to my model or seeding methods ...

Base Entity:

public class BaseEntity
{
    public int ID { get; set; }
}

Customer:

public class Customer:BaseEntity
{
    public string Name { get; set; }
    public Language Language { get; set; }
    public ICollection<Address> Addresses { get; set; }
}

Language:

public class Language : BaseEntity

{
    public string Name { get; set; }
    public string LanguageCode { get; set; }

    [Required]
    public ICollection<Customer> Customers { get; set; }
}

Address:

public class Address : BaseEntity
{
    public Customer Customer { get; set; }
}

Seeding method:

  Language newLanguageNL = new Language("Dutch");
  newLanguageNL.ID = 1;

  Language newLanguageFR = new Language("French");
  newLanguageFR.ID = 2;

  Language newLanguageEN = new Language("English");
  newLanguageEN.ID = 3;

  ctx.Languages.Add(newLanguageNL); 
  ctx.Languages.Add(newLanguageEN); 
  ctx.Languages.Add(newLanguageFR);

  Address addressBE = new Address("informatica laan", "10", "bus nr 1", "8900", "België");
  addressBE.ID = 1;

  Address addressBE2 = new Address("rue de l'informatique", "20", "boite nr 2", "7780", "Belgique");
  addressBE2.ID = 2;

  Address addressEN = new Address("techstreet", "30", "box nr 1", "4000", "Bulgaria");
  addressEN.ID = 3;

  ctx.Addresses.Add(addressEN);
  ctx.Addresses.Add(addressBE);
  ctx.Addresses.Add(addressBE2);

  Customer newCustomer = new Customer("Customer name", newLanguageNL, addressBE);
  // ctx.Customers.AddOrUpdate(c => c.Name, newCustomer);
  ctx.Customers.Add(newCustomer);

  base.Seed(ctx);

OnModelCreating:

     protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);      

        // setting the Product FK relation required + related entity
        modelBuilder.Entity<Entity.ProductSupplierForContract>().HasRequired(psfc => psfc.Product)
                                                            .WithMany(p => p.ProductSupplierForContracts)
                                                            .HasForeignKey(psfc => psfc.Product_Id);

        // setting the Supplier FK relation required + related entity
        modelBuilder.Entity<Entity.ProductSupplierForContract>().HasRequired(psfc => psfc.Supplier)
                                                           .WithMany(s => s.ProductSupplierForContracts)
                                                           .HasForeignKey(psfc => psfc.Supplier_Id);

        // setting the Contract FK relation required + related entity
        modelBuilder.Entity<Entity.ProductSupplierForContract>().HasOptional(psfc => psfc.Contract)
                                                          .WithMany(c => c.ProductSupplierForContracts)
                                                          .HasForeignKey(psfc => psfc.Contract_Id);


        modelBuilder.Entity<Entity.PurchasePrice>()
     .ToTable("PurchasePrices");

        modelBuilder.Entity<Entity.SalesPrice>()
    .ToTable("SalesPrices");

        // Bundle in Bundle
        modelBuilder.Entity<Entity.Bundle>().HasMany(b => b.ChildBundles);                      
    }

Can anyone help me on this one please, thank you in advance for any feedback. I have tried using AddOrUpdate() with no luck.

I see a few issues with your code. By convention, an int column called ID is going to be an identity column so you can't set it's ID explicitly without issuing a SET IDENTITY_INSERT Language ON (unless you have fluent code overriding this).

AddOrUpdate is intended for these situations. You have not shown that code. Another way is shown below:

...
if (!ctx.Languages.Any(l => l.ID == 1))  // Check if already on file
{
    ctx.Database.ExecuteSqlCommand("SET IDENTITY_INSERT Language ON");  // Omit if not identity column
    var dutch = new Language {
        ID = 1,
        Name = "Dutch",
        Code = "NL"
    };
    ctx.Languages.Add(dutch);
    ctx.SaveChanges();
    ctx.Database.ExecuteSqlCommand("SET IDENTITY_INSERT Language OFF"); // Omit if not identity column
}
... repeat for other languages
... similar code for other seeded tables