MessagingCenter.Subscribe Multiple Call

I found my code Multiple call ,anyone who can help me find the reason

           MessagingCenter.Subscribe<SelectCitiesViewModel,string> (this, "Province", (sender, arg) => {
                Province = arg;
                MessagingCenter.Send (this, "Dismiss", sender);
            });

            static int i;
            MessagingCenter.Subscribe<CityViewModel> (this, "Dismiss", (sender) => {
                Navigation.RemovePage (this);
                Debug.WriteLine (sender.city_name + "excuted" + (i++));
            });

Here is the Application Output
2015-03-21 11:18:03.199 TescarManage.iOS[7332:466564] 重庆excuted0
重庆excuted0
2015-03-21 11:18:17.073 TescarManage.iOS[7332:466564] 北京excuted1
北京excuted1
2015-03-21 11:18:17.075 TescarManage.iOS[7332:466564] 北京excuted2
北京excuted2
2015-03-21 11:18:17.077 TescarManage.iOS[7332:466564] 北京excuted3
北京excuted3
2015-03-21 11:18:29.434 TescarManage.iOS[7332:466564] 随州excuted4
随州excuted4
2015-03-21 11:18:29.435 TescarManage.iOS[7332:466564] 随州excuted5
随州excuted5
2015-03-21 11:18:29.436 TescarManage.iOS[7332:466564] 随州excuted6
随州excuted6
2015-03-21 11:18:29.437 TescarManage.iOS[7332:466564] 随州excuted7
随州excuted7
2015-03-21 11:18:29.438 TescarManage.iOS[7332:466564] 随州excuted8
随州excuted8
2015-03-21 11:18:29.439 TescarManage.iOS[7332:466564] 随州excuted9
随州excuted9
2015-03-21 11:18:39.810 TescarManage.iOS[7332:466564] 黔西南excuted10
黔西南excuted10
2015-03-21 11:18:39.811 TescarManage.iOS[7332:466564] 黔西南excuted11
黔西南excuted11
2015-03-21 11:18:39.811 TescarManage.iOS[7332:466564] 黔西南excuted12
黔西南excuted12
2015-03-21 11:18:39.812 TescarManage.iOS[7332:466564] 黔西南excuted13
黔西南excuted13
2015-03-21 11:18:39.813 TescarManage.iOS[7332:466564] 黔西南excuted14
黔西南excuted14
2015-03-21 11:18:39.813 TescarManage.iOS[7332:466564] 黔西南excuted15
黔西南excuted15
2015-03-21 11:18:39.814 TescarManage.iOS[7332:466564] 黔西南excuted16
黔西南excuted16
2015-03-21 11:18:39.815 TescarManage.iOS[7332:466564] 黔西南excuted17
黔西南excuted17
2015-03-21 11:18:39.816 TescarManage.iOS[7332:466564] 黔西南excuted18
黔西南excuted18
2015-03-21 11:18:39.817 TescarManage.iOS[7332:466564] 黔西南excuted19
黔西南excuted19