JDeveloper has an integrated WebLogic Server now - and when you deploy the applications to it, it behaves a little different then in the old days of OC4J. For example you can have multiple applications deployed and running at the same time on the server.
However, sometimes one of those application might still be around when you don't need it anymore. This can happen for example if you had your WLS running and killed it with the task manager.
This might cause an error when you try and run the application again - saying that the application is already up there.
The nice thing in JDeveloper is that you can actually undeploy that application directly from inside JDeveloper. Here is how:
From the View menu choose Application Server Navigator
Expend the IntegratedWLSConnection to look at the applications that are available.
(By the way, you should see there all the ADF libraries, DO NOT UNDEPLOY THEM)
Next you can locate your application, right click it and undeploy it.
You should now be able to redploy it.