WebLogic 10 is JEE-compliant. One of the differences between the older J2EE and JEE is that in JEE, JSF is an integral part of any full-stack server implementation (such as WebLogic).
Because of this, you should not include the jsf-impl.jar as part of your WAR as it will conflict with WebLogic's built-in jsf-impl. You would only include jsf-impl.jar in a WAR for an incomplete JEE implementation such as Tomcat.
Sometimes, however, the JSF implementation that's built into the server isn't appropriate and you need to override it. For information on that, you'll have to consult the proper WebLogic documentation, since the requirements are dependent on the server itself.
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