Android SQLite Error 19

I am in trouble with error 19 (android.database.sqlite.SQLiteConstraintException: error code 19: constraint failed) and I saw that this happens the second time I run the database.

Logcat :

04-05 23:17:34.409  17818-17818/database.database E/Database﹕ Error inserting friends
    android.database.sqlite.SQLiteConstraintException: error code 19: constraint failed
            at android.database.sqlite.SQLiteStatement.native_execute(Native Method)
            at android.database.sqlite.SQLiteStatement.execute(SQLiteStatement.java:61)
            at android.database.sqlite.SQLiteDatabase.insertWithOnConflict(SQLiteDatabase.java:1584)
            at android.database.sqlite.SQLiteDatabase.insert(SQLiteDatabase.java:1428)
            at database.database.DatabaseHandler.addFriend(DatabaseHandler.java:64)
            at database.database.MainActivity.onCreate(MainActivity.java:20)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
            at android.app.ActivityThread.access$1500(ActivityThread.java:117)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:130)
            at android.app.ActivityThread.main(ActivityThread.java:3687)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:878)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:636)
            at dalvik.system.NativeStart.main(Native Method)
04-05 23:17:34.409  17818-17818/database.database E/Database﹕ Error inserting friends
    android.database.sqlite.SQLiteConstraintException: error code 19: constraint failed
            at android.database.sqlite.SQLiteStatement.native_execute(Native Method)
            at android.database.sqlite.SQLiteStatement.execute(SQLiteStatement.java:61)
            at android.database.sqlite.SQLiteDatabase.insertWithOnConflict(SQLiteDatabase.java:1584)
            at android.database.sqlite.SQLiteDatabase.insert(SQLiteDatabase.java:1428)
            at database.database.DatabaseHandler.addFriend(DatabaseHandler.java:64)
            at database.database.MainActivity.onCreate(MainActivity.java:21)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
            at android.app.ActivityThread.access$1500(ActivityThread.java:117)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:130)
            at android.app.ActivityThread.main(ActivityThread.java:3687)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:878)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:636)
            at dalvik.system.NativeStart.main(Native Method)
04-05 23:17:34.419  17818-17818/database.database I/Name:﹕ id :1  descr :descr1
04-05 23:17:34.419  17818-17818/database.database I/Name:﹕ id :2  descr :descr2
04-05 23:17:34.419  17818-17818/database.database I/Name:﹕ id :3  descr :descr3
04-05 23:17:34.419  17818-17818/database.database I/Name:﹕ id :4  descr :descr4

Database handler:

@Override
public void onCreate(SQLiteDatabase db) {
    String CREATE_TABLE = "CREATE TABLE IF NOT EXISTS " + TABLE_FRIENDS
            + "("
            + KEY_ID                     + " TEXT PRIMARY KEY,"
            + A               + " TEXT"
            + ")";


    db.execSQL(CREATE_TABLE);
}

void addFriend(Friend_Object friend) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues values = new ContentValues();
    values.put(KEY_ID, friend.ID());
    values.put(A, friend.A());

    db.insert(TABLE_FRIENDS, null, values);
    db.close();
}