When you deploy or debug an app for SharePoint, you will get multiple user credential challenge [in my case I am using NTLM authentication] but fail to login. This is caused by the loopback check feature exists since Windows Server 2003 SP1. To disable this loopback check, open up powershell and execute the following command. Note: Do this only on your development machine.
New-ItemProperty HKLM:\System\CurrentControlSet\Control\Lsa -Name "DisableLoopbackCheck" -Value "1" -PropertyType dword