SQLite DB is Duplicating a last record in Table

how to prevent a SQLite DB to duplicate my record its actually duplicating a last record of my Table i have primary key don't want to make it UNIQUE Key.

public void AddCity(String cityname)
{
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues _values = new ContentValues();

    try {

        _values.put("City_name", Cityname);
        db.insert(TABLE_City,null,_values);
        // Log.d("Record inserted",Cityname);
        db.close();
    }
    catch (SQLException e)
    {
        e.printStackTrace();
        Log.d("Exception", e.toString());

    }
}