场景：

logger.Info("f: " + wx.From);
logger.Info("c: " + wx.Code);
logger.Info("s: " + wx.State);

Response.Redirect("https://open.weixin.qq.com/connect/oauth2/authorize?appid=8RAN90uJ1967wpt1Y5&redirect_uri=www.xxxx.com%3a8001%2fWxAuthPage.aspx?from=notreserve&response_type=code&scope=snsapi_base&state=1#wechat_redirect");

出现：

除了code不一致，其它一致。

这样导致如下两次插入：

var mealtake = new MealTake
{
Id = Guid.NewGuid(),
IsTaked = true,
TakeDate = DateTime.Now,
MealType = mt.MealType,
PersonOrder = personOrder
};

context.Set<MealTake>().Add(mealtake);
context.SaveChanges();

想到的解决方法：

Response.Redirect("https://open.weixin.qq.com/connect/oauth2/authorize?appid=8RAN90uJ1967wpt1Y5&redirect_uri=www.xxxx.com%3a8001%2fWxAuthPage.aspx?from=notreserve&response_type=code&scope=snsapi_base&state=1#wechat_redirect");

改成

Response.Redirect("https://open.weixin.qq.com/connect/oauth2/authorize?appid=8RAN90uJ1967wpt1Y5&redirect_uri=www.xxxx.com%3a8001%2fWxAuthPage.aspx?from=notreserve&response_type=code&scope=snsapi_base&state="+Guid.NewGuid().ToString()+"#wechat_redirect");

再把插入代码改成

if (!context.Set<PersonOrder>().Any(n => n.Comment == wx.State))
{
　　var person = context.Set<MealPerson>().Include("PersonType").FirstOrDefault(n => n.Id == wxUserInfo.User.Id);
　　var personOrder = MealOrderService.CreatePersonOrder(person, mt.MealType, mealdate, false);
　　personOrder.Comment = wx.State;
　　var mealtake = new MealTake
　　{
　　　　Id = Guid.NewGuid(),
　　　　IsTaked = true,
　　　　TakeDate = DateTime.Now,
　　　　MealType = mt.MealType,
　　　　PersonOrder = personOrder
　　};

　　context.Set<MealTake>().Add(mealtake);
　　context.SaveChanges();
}

=============================================================

具体原因，原来是redirect_uri导致的，漏了对redirect_uri进行完整的UrlEncode：

redirect_uri=www.xxxx.com%3a8001%2fWxAuthPage.aspx?from=notreserve

改成

redirect_uri=www.xxxx.com%3a8001%2fWxAuthPage.aspx%3ffrom%3dnotreserve

就不会发生两次回调了