EF6 Cannot Delete/Update - An object with the same key already exists in the ObjectStateManager - entity-framework

Question

I am using EF 6 Code First and I need to delete an item and then also update a different item within a collection of Entities. If I try to delete one item and then modify a completely different item I get the error message "An object with the same key already exists in the ObjectStateManage" This is inaccurate because there are two objects with completely different PK IDs but when the update happens it throws the error. If I comment out the code to delete then the update works just fine with multiple items to update. Why would it complain about the "same key" when the keys are different?
foreach (var phone in phones)
{
if (!_isValidPhone(phone))
{
if(phone.PhoneId != 0)
{
var deletePhone = _db.Phones.FirstOrDefault(r => r.PhoneId == phone.PhoneId);
_db.Entry(deletePhone).State = EntityState.Deleted;
continue;
}
}
if (_isNewPhone(phone))
{
AddNewPhone(phone, _person);
}
else
{
UpdatePhoneData(phone, _person.Phones.FirstOrDefault(r => r.Order == phone.Order));
}
}
private void UpdatePhoneData(Phone phoneFrom, Phone phoneTo)
{
phoneTo.Note = phoneFrom.Note;
phoneTo.PhoneNumber = phoneFrom.PhoneNumber;
phoneTo.Order = phoneFrom.Order;
_db.Entry(phoneTo).State = EntityState.Modified;
}

Answer 1

If a phone is not valid and has id you try to add it to the context in two ways:
While deleting it:
_db.Entry(deletePhone).State = EntityState.Deleted;
Besides, when checking if it's new or not you either add or update it. Thats the problem.
So, what you need to do is wrap the add or update part inside an else, to add or update only if the phone has not been deleted.

Answer 2

This is not really a logic issue, the phone that was being updated is completely different than the phone that was being deleted. The issue is in the object statemanager. Because I was doing this in the delete
var deletePhone = _db.Phones.FirstOrDefault();
And then later I had a separate list of Phones where I try to set one of them to modified
_db.Entry(phoneTo).State = EntityState.Modified;
Well the object state manager now has each phone loaded twice basically. So if I just use the _person.Phones for both my delete and modify, then the Phones list is only loaded once and there are now duplicate keys.
_db.Entry(_person.Phones.FirstOrDefault(r => r.PhoneId == phone.PhoneId)).State = EntityState.Deleted;