DAG creation error

I'm trying to set up my first DAG.

Installed all pre-req's
Added replication NICS, tested ping
Gave Exchange Trusted Subsystem permissions to member server

DAG is created fine. When I add 1 Exchange server I get the error:

Summary: 1 item(s). 0 succeeded, 1 failed.
Elapsed time: 00:00:06


ECS-EX
Failed

Error:
A server-side database availability group administrative operation failed. Error: The operation failed. CreateCluster errors may result from incorrectly configured static addresses. Error: Computer account 'DAG1' couldn't be validated by user 'NT AUTHORITY\SYSTEM'. Error: An attempt to use the specified cluster name failed because an enabled computer object with the given name already exists in the domain [Server: ECS-EX.evo.local]

A server-side database availability group administrative operation failed. Error: Computer account 'DAG1' couldn't be validated by user 'NT AUTHORITY\SYSTEM'. Error: An attempt to use the specified cluster name failed because an enabled computer object with the given name already exists in the domain
Click here for help... http://technet.microsoft.com/en-US/library/ms.exch.err.default(EXCHG.140).aspx?v=14.1.323.0&t=exchgf1&e=ms.exch.err.ExC9C315

Warning:
The operation wasn't successful because an error was encountered. You may find more details in log file "C:\ExchangeSetupLogs\DagTasks\dagtask_2011-09-08_15-19-55.016_add-databaseavailabiltygroupserver.log".


Exchange Management Shell command attempted:
Add-DatabaseAvailabilityGroupServer -MailboxServer 'ECS-EX' -Identity 'DAG1'

Elapsed Time: 00:00:06