WebLogic error explained: "DescriptorException... Unmarshaller failed... MarshallerFactory... BeanAlreadyExistsException"

If you get the following error when trying to deploy an EAR to WebLogic (11g version 10.3.2):

Unable to load descriptor... The error is weblogic.descriptor.DescriptorException: Unmarshaller failed
at weblogic.descriptor.internal.MarshallerFactory$1.createDescriptor(MarshallerFactory.java:161)
...
Caused by: com.bea.xml.XmlException: weblogic.descriptor.BeanAlreadyExistsException: Bean already exists: "weblogic.j2ee.descriptor.ModuleBeanImpl...

...it indicates that you are trying to deploy multiple WARs (e.g. 2 WARs) with the same context-root. That is not allowed. Either consolidate the WARs or give one a different context-root.