来源：http://ericphan.net/blog/2016/6/10/solving-the-tfs-build-agent-error-the-session-for-this-agent-already-exists
Recently we've been getting the following error for our TFS build agents which caused all our builds to fail
Figure: TFS Builds are failing
The error reported is "The job has been abandoned because agent."
Investigating the _diag folder of the build agent itself reveals the actual error.
--------------------------------------------------------------------------- The session for this agent already exists. Sleeping for 30 seconds before next retry. Attempts=1/10. --------------------------------------------------------------------------- Microsoft.TeamFoundation.DistributedTask.WebApi.TaskAgentSessionConflictException: The task agent Agent-SYDTFSBLDP01-1 already has an active session for owner SYDTFSBLDP01. at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.HandleResponse(HttpResponseMessage response) at Microsoft.VisualStudio.Services.WebApi.VssHttpClientBase.
To fix this issue, you need to re-run the ConfigureAgent.cmd and on the first option "Do you want to also replace the server registration (default is N)" Choose "Y"
（这里我遇到的情况略有不同，我用到的ConfigureAgent文件的后缀是ps1，即是powershell文件，故需要以管理员权限打开powershell，然后运行ConfigureAgent）
PS C:\Agents\2> .\ConfigureAgent.cmd An existing configuration file was detected. This will update the local agent settings. Do you want to also replace th e server registration (default is N)? Y
This will remove the existing agent registration and clean up the locked "session".