EF多对多，更新时总是在桥表中插入新行

Question

   `<?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout 
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:paddingBottom="@dimen/activity_vertical_margin"
     android:paddingLeft="@dimen/activity_horizontal_margin"
     android:paddingRight="@dimen/activity_horizontal_margin"
     android:paddingTop="@dimen/activity_vertical_margin"
     app:layout_behavior="@string/appbar_scrolling_view_behavior"
     tools:context="com.cancunsteve.aboutcancunsteve.MainActivity"
     tools:showIn="@layout/activity_main">


    <ScrollView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/scrollView"
    android:layout_alignParentStart="true"
    android:layout_marginTop="56dp" />
    <ImageView android:id="@+id/imageView"
    android:layout_column="1"
    android:background="@mipmap/lpussy"
    android:contentDescription='android:ContentDescription="@string/desc"'
    tools:ignore="HardcodedText"
    android:layout_width="200dp"
    android:layout_height="230dp" />


    <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="serving good people with special deals. "
    android:id="@+id/text1"
    android:layout_alignTop="@+id/imageView"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:background="#ffffff"
    android:foreground="#05ffffff"
    android:foregroundTint="#03ffffff"
    android:textColor="#000000" />



    <EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/editText2"
    android:text="Cancun, Playa del Carmen, Chichen Itza."
    android:layout_below="@+id/text1"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true" />

    <EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/editText"
    android:text="Holbox, Taxco, Palenque, Acapulco, Guanajuato,  
    Monterrey, Tenacatita, Mascota"
    android:layout_below="@+id/editText2"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<ImageButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageButton"
    android:layout_below="@+id/editText3"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:background="@mipmap/campa"
    tools:ignore="ContentDescription"
    android:layout_alignBottom="@+id/editText5" />

    <EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:inputType="textEmailAddress"
    android:ems="10"
    android:id="@+id/editText3"
    android:text="thewebmaster@cancunsteve.com"
    android:layout_below="@+id/editText"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignRight="@+id/editText"
    android:layout_alignEnd="@+id/editText" />

    <EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:inputType="phone"
    android:ems="10"
    android:id="@+id/editText4"
    android:text="       011 52 998 8873919"
    android:layout_below="@+id/editText3"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_marginLeft="42dp"
    android:layout_marginStart="32dp" />

    <EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/editText5"
    android:text="be sure to visit us for discounts on your hotel bookings"
    android:layout_below="@+id/editText4"
    android:layout_toRightOf="@+id/scrollView"
    android:layout_toEndOf="@+id/scrollView" />

    <TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="click buttons multiple times to change state"
    android:id="@+id/textView2"
    android:layout_below="@+id/imageButton" />


   </RelativeLayout>`

// ----------------

class Student{
public string Name {get; set;}

public EntityCollection<Info> Infos {get; set;}
}

class Info{
public string Title {get; set;}

public EntityCollection<Student> Students {get; set;}
}

在数据库中，我有三个表（Student，Info和桥表StudentInfo）。插入时可以正常工作。但是，当我尝试获取Student实体及其信息时，我设置了新的Student Name并保存了更改数据库上下文。它总是将新记录插入桥表（重复的StudentId和InfoId）。如何避免呢？