Solution

Deploying a WAR Module in Sun GlassFish Enterprise Server v2.1.1 Application Deployment Guide states:

Web module context roots must be unique within a server instance.

While technically accurate, it is useful to add the following further clarification to this statement:

The DAS in Sun GlassFish Enterprise Server versions v2.1.1 and later supports the deployment of multiple web applications using the same web context root as long as those applications are deployed to different Enterprise Server instances. Deploying multiple applications using the same context root within a single Enterprise Server instance will produce a DAS error.