A deployment or retraction is already under way for the solution "xxx.wsp", and only one deployment or retraction at a time is supported
it occurs if anomalies deployment is running for the same package in the backend due to some previous deployment has been crashed before for some reason.
To avoid this problem we need do the followings:
Look for the deploy jobId to break the running by the following command: C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN>stsadm-o enumdeployments
Cancel the deploy typing the following command: C:\Program
Files\Common Files\Microsoft Shared\web server
extensions\12\BIN>stsadm-o canceldeployment-id "GUID jobId"
But after execute the second command I got following message The job 'GUID' is already running and cannot be canceled.