SQL Server 2012 Availability Group issue
You have to do following :


Drop the availability group then 
using regedit, (first backup the registry of old availability group that starts with HKEY_LOCAL_MACHINE\Cluster\HadrAgNameToldMap) and then delete that key and  
Finally create a new Availability Group with new name.


Note: Extra Caution .. as you are dealing with Windows Registry, Back it up first