Here is something that I figured out as I got this error "The Mendix Cloud is already deploying this app. Please wait for the current deployment to finish before attempting a new one" numerous times.
1. First of all if you should "wake up" the app from sleep before deploying a new version of it. . Ususaly the mendix modeler does this when deploying but, you know things happen. So it is best to wake the application by loging on to the application URL with your browser, and mess around here and there in the application.
2. Then you can deploy the new versions and updates from the mendix modeler.
3. Be sure there are no interupptions in your internet connection while deploying new revisions to the cloud untill you get the message "you application was sucessfuly deployed"
I found out that if the connection drops, when deploying, some how the status of deploying to the cloud doesn't reset. The mendix sandbox somehow has an error in resuming deployment. Hope the mendix team will resolve this error soon. OR atleast put a button or something to "deploy new" starting the deployment from the begening.
4. I found out that, even without contacting a support ticket, the deployment ususally resets within 24 hours. So that it becomes available to redeployment.
All of the above are my experiences and because this has happned to me many times, and I have contacted the support team too many a time. But they also ssaid that the user can't avoid this from happening and they will resolve this error soon from their side.
Hope this helps you guys..