What I mean is that for a particular key, more than one value has been set within the worker-local SPARK_JAVA_OPTS.  So, for example: SPARK_JAVA_OPTS="-Dspark.local.dir=/a/path -Dspark.local.dir=/another/path".  I let identical values associated with duplicate keys pass without comment (after eliminating the duplication in the Set), so the error being caught is more than just that there are multiple instances of a particular key.

If you've got clearer but still concise wording to express that error condition, I'll take it.