title: Glassfish 4 deployment error
body: <p>I added some libraries to my JavaEE project and then deploy the application works. I modified something and I got this error message in Eclipse:</p>
  
  <pre><code>Cannot Deploy ProjectName
  deploy is failing=Error occurred during deployment: Keys cannot be duplicate. 
  Old value of                 this key property, nullwill be retained. 
  Please see server.log for more details.
  java.lang.LinkageError: loader (instance of  
  org/glassfish/web/loader/WebappClassLoader):
  attempted  duplicate class definition for name: 
  "org/glassfish/web/loader/JdbcLeakPrevention"
  loader (instance of  org/glassfish/web/loader/WebappClassLoader): attempted  duplicate 
  class definition for name: "org/glassfish/web/loader/JdbcLeakPrevention"
  </code></pre>
  
  <p>I made some research and I found out that it is not an Eclipse bug. I'm working with Kepler but I tested with Juno too. Got the same error message. 
  I downloaded Glassfish 3.1.2.2 and the message gone. </p>
  
  <p>Why am I getting this message?</p>