Duplicate managed accounts - Item has already been added error

You can add multiple managed accounts in SharePoint but you cannot delete them. If you add same account twice, and you are logged in with that account, you get following error when you try to do anything in the central admin:

Item has already been added, key in dictionary: domain\user

The solution is to remove one managed account. Use PowerShell.

$managedaccount = get-spmanagedaccount -identity "domain\user"

remove-spmanagedaccount $managedaccount[0]

This will remove the extra managed account. You may have to log out and log in again. Also recycle the application pool associated with that managed account.