Multiple ES-Hadoop versions detected - false positive

[frensjan]

org.elasticsearch.hadoop.util.Version throws a RuntimeException if it finds the class itself multiple times on the class-path:

Multiple ES-Hadoop versions detected in the classpath; please use only one
[list of jars containing the Version class]

This probably is a great help in cases where issues arise from actually having different versions on the class-path. However, it actually doesn't check for whether there actually are different versions.

When deploying with Flink we run into the issue that ES Hadoop is added to the class-path twice. Once because it is packaged in a user-lib, but it is also added to the class-path for YARN deployments. This is unfortunate, not that easy to work around, especially because it also influences a lot of other class-path issues.

I'd like to provide a PR which actually checks if there are multiple different versions on the class-path.

I'm thinking about something like:

Set versions = get versions
if(versions.size() > 1) {
   throw ...
}

instead of the current implementation of

List jars = get normalized jars containing `org.elasticsearch.hadoop.util.Version`
if(jars.size() > 1) {
   throw ...
}

Any (early) feedback is more than welcome!