It's not that easy, see https://github.com/elastic/elasticsearch/issues/13604
In Java, it is perfectly valid to have two identical classes on the class path, because only the first one will be loaded. This is part of the Java Language Specification, implemented by JVM caching.
ES is checking for duplicate class names. It does not detect identical classes by checking the binary representations of the class. So the check is too strict. It's the binary representation the JVM caches, not the class name (which is given by classloader and binary name of class).
From the Java Language specification http://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.2.1
In particular, a class loader may cache binary representations of classes and interfaces, prefetch them based on expected usage, or load a group of related classes together. These activities may not be completely transparent to a running application.
By binary representation caching, the JVM ensures that identical classes are loaded only once during the JVM lifetime, no matter what class loader is in use. That's also the reason why the method
findLoadedClass exists, which can be used to check for a class being in the cache and loaded by this class loader or not https://docs.oracle.com/javase/8/docs/api/java/lang/ClassLoader.html#findLoadedClass-java.lang.String-