[logback-user] Why is Logback being configured twice?

I'm trying to switch an application from from log4j to Logback. This app uses SLF4J, Spring Framework and is deployed in Tomcat 7. Spring Framework uses JCL, so I added the following JARs to the application's classpath: slf4j-1.6.1.jar jcl-over-slf4j-1.6.1.jar logback-classic-0.9.29.jar logback-core-0.9.29.jar I created a logback.xml file, with debug="true" so I could debug my configuration. When I deploy this webapp in Tomcat, I can see that Logback is being configured twice. Why would this happen? Thanks in advance, Daniel Serodio