c# - System.Web.HttpContext.Current.User.Identity.Name giving other user's name -

i using windows authentication intranet application developed in mvc 4.0.

to current user using below code.

string userid = system.security.principal.windowsidentity.getcurrent().name.toupper(); 

in web.config file, set authentication mode="windows" , in iis enabled windows authentication , disabled ananymous.

its working fine in local when deployed application windows server 2012 , iis 8.5 ,am facing 2 issues unable fix.

first issue when user browse application on machine should logged on user same fine 1 user. suppose 2 users browsing url @ same time displaying same name both users, random , displaying one's name of both 2 users. have not used session or caching or cookies in entire application.

second issue randomly users getting credentials popup , when give correct username , password not worked.issue random not sure when get.

request on these issues, tried lot didn't find solution.