When a user is Logged in, and goes to Login Page while so. If he tries to login again you get Anti forgery Error.
The anti-forgery token could not be decrypted. If this application is hosted by a Web Farm or cluster, ensure that all machines are running the same version of ASP.NET Web Pages and that the configuration specifies explicit encryption and validation keys. AutoGenerate cannot be used in a cluster.
Another type of error I get is:
The provided anti-forgery token was meant for a different claims-based user than the current user.
How to handle this Anti forgery Errors?
Create action filter inhering HandleErrorAttribute as following example. Then you can check the request and handle the error.
This is my example, remember you have to handle your redirect sections depends on your request and requirements.
Then the login
Edited for comment
Use another controller / action as AlreadySignIn()
Controller code
Razor View
Hope this helps.