ServiceLoader no longer detects duplicates on the class path

We have a small regression in ServiceLoader in JDK 9 whereby it no longer detects duplicate providers on the class path. The issue is specific to the case where there are two JAR files with service configuration files that list the same provider class.

More details here:

http://mail.openjdk.java.net/pipermail/core-libs-dev/2017-March/046832.html