From BZ:
"In case when some login module should use password stacking then value of password-stacking option should be set to useFirstPass. All login modules should respect it. However implemetation of org.jboss.security.ClientLoginModule uses password-stacking differently - it uses password stacking everytime when some value is set for password-stacking option (even value false). It should work same as other login modules. Current behavior can be confusing and can lead to incorrectly set server configuration."