test env:
jre8 b28, winxp

steps to reproduce:
1) download following jnlp to local dir like c:
http://sqeweb.sfbay.sun.com/jsn/users/fred/reproduce/overwritecodebase/jnlp/BAT_simple.jnlp
2) import it by "javaws -import -codebase http://sqeweb.sfbay.sun.com/jsn/users/fred/reproduce/overwritecodebase/jnlp file:///c:/BAT_simple.jnlp"
if app can't be imported due to following error, bug is reproduced.
Unable to load resource: http://sqeweb.sfbay.sun.com/jsn/users/fred/reproduce/ovewritecodebase/jnlp/sses/hellojaws.jar

if download another jnlp, http://sqeweb.sfbay.sun.com/jsn/users/fred/reproduce/overwritecodebase/BAT_simple.jnlp and import it by "javaws -import -codebase http://sqeweb.sfbay.sun.com/jsn/users/fred/reproduce/overwritecodebase file:///c:/BAT_simple.jnlp", the app can be imported successfully.