The error “Item has already been added. Key in dictionary: ‘domain\account’ Key being added: ‘domain\account'” was preventing access to the configure managed accounts and configure service account menu’s within Central Administration, Security, General Security.
The solution is to remove one managed account using PowerShell.
$managedaccount = get-spmanagedaccount -identity “domain\user”
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.
Core Solutions of Microsoft SharePoint 2013
MOC 20331B official course
This is a great course and I would thoroughly recommend it if you are looking to implement SharePoint 2013 or have already deployed it.
I did this course at QA, International House, London. The facilities were fantastic with lots of lunch venues within 5 mins away.
I have to mention the course trainer Richard Keep http://uk.linkedin.com/pub/richard-keep/22/688/856, who is one of the best trainers I have ever experienced. He has an in depth knowledge of a variety of IT platforms and solutions and delivered the course in such a way that enabled me to learn and understand whilst enjoying the material and having fun.