Can be seen with M5, also with Snapshot as of 2010-10-10.
This might have the same root cause as JBAS-6335, but JBAS-6335 is reported to be fixed, whereas this issue definitely exists.

If you have 2 or more files in the deploy folder, which have the same name with different suffixes, the deployer will run into the following error (full log will get attached):
[AbstractKernelController] Error installing to Real: name=vfs:///mnt/san/develop/nbr/XXX/jboss/xxx/deploy/MyApp.war state=PreReal mode=Manual requiredState=Real: org.jboss.deployers.spi.DeploymentException: Error deploying: jboss.naming:application=MyApp
Caused by: java.lang.IllegalStateException: jboss.naming:application=MyApp is already installed.

e.g. if you have an WAR and a SAR which have the same name:

• MyApp.sar
• MyApp.war
  or the same with war and probably any other combination of it:
• MyApp.ear
• MyApp.sar

Workaround is to rename one of the files, which leads to successful deployment.