ASP.NET MVC and Entity Framework. How to avoid duplicate rows?

So I'm new to ASP.NET MVC and I'm trying to implement an address manager.I'm using Linq2Entity Framework. I've got a table with contacts and one with phone numbers. These two tables are linked by a contact_has_phone_number table. The Entity Framework enables me to get all phone numbers that are assigned to one contact. This is done by calling contact.PhoneNumbers which returns a collection of phone numbers. A new number is added with contact.PhoneNumber.Add(number). The Problem is that I am getting duplicate phone number entries in the phone number table. What I would like to get is that an existing phone number only gets linked in the contact_has_phone_number table. For Example:

Table "contact"

ID_Contact  | First_Name |  Last_Name  
1          开发者_如何学Go |   Jeff     |    Bridges  
2           |   Peter    |    Miller  

Table "contact_has_phone_number"

ID_Contact  | ID_Number  
1           |    1  
1           |    2  
2           |    1  

Table "phone_number"

ID_Number  | Number  
1          | 1234567  
2          | 7654321  

At the momonet I would get 3 rows in the "phone_number" table with 2 rows with identical numbers.

ID_Number  | Number  
1          | 1234567  
2          | 7654321  
3          | 1234567  

Would be a blast if you could help me with this.

This is a many to many situation. I have encountered a situation like this (not contact related) and this is what I did.

In creating a new contact, I will have the contact details as well as the phone numbers. the below code can be used

public void AddNewContact(Contact contact)
{
    var storedPhones = context.PhoneNumbers.Select(s => s).ToList();
            foreach (var s in contact.PhoneList)
            {
                var p = s.PhoneNumber; 
                var storedPhone = storedPhone s.Where(f => f.PhoneNumber == p).Select(t => t).FirstOrDefault();
                if (storedPhone == null)
                {
                    var newPhoneNumber = MySqlEFModel.PhoneNumber.CreatePhoneNumber(Guid.NewGuid(), p);
                    context.AddToPhoneNumbers(newPhone);
                    contact.PhoneNumbers.Add(newPhone);
                }
                else
                {
                    contact.PhoneNumbers.Add(storedPhone);
                }
            }
        context.AddToContacts(contact);
        context.SaveChanges();
}

EDIT:

I should have done this at the beginning of the method, before the loop.

                    var newContact = MySqlEFModel.Contact.CreateContact(keyfields, ... ... , ...);

Then, the lines

contact.PhoneNumbers.Add(storedPhone);

will become

newContact.PhoneNumbers.Add(storedPhone);

I had a very similar problem with duplicate records occurring using EF. After much mess, I finally discovered the solution:

You must always attach newly created entities to a table before executing .SaveChanges()

My issue was that SaveChanges was being called before attaching a new entity I created to a table so there was one entity created after SaveChanges was called and another duplicate entity called after AddObject();SaveChanges(); was run.

TL;DR:

Result with duplicate records:

Pro.Machine = new XXXDB.Machine();
Pro.Machine.Translate(machine); // SaveChanges() was being called by this method.
XXXDB.XXXDB.DB.Machines.AddObject(Pro.Machine);
XXXDB.XXXDB.DB.SaveChanges();

was changed to:

Pro.Machine = new XXXDB.Machine();
XXXDB.XXXDB.DB.Machines.AddObject(Pro.Machine);
Pro.Machine.Translate(machine); //SaveChanges() occurs after attaching to the machines table.
XXXDB.XXXDB.DB.SaveChanges();

Note: database names changed to protect privileged client.