objc[970]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/bin/java and /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/libinstrument.dylib. One of the two will be used. Which one is undefined.
在Mac上装了Intellij Idea学习Kotlin，随手用IJ带的Kotlin Java工程模板创建了一个测试工程，控制台报了这样一个Error，意思是这个JavaLaunchHelper类被实现了两次。无奈之下搜索了万能的Stack Overflow，找到了解决方案。
链接如下：http://stackoverflow.com/questions/43003012/objc3648-class-javalaunchhelper-is-implemented-in-both
引入最高票的回答如下：
You can find all the details here:
IDEA-170117 "objc: Class JavaLaunchHelper is implemented in both ..." warning in Run consoles
It's the old bug in Java on Mac that got triggered by the Java Agent being used by the IDE when starting the app. This message is harmless and is safe to ignore. Oracle developer's comment:
The message is benign, there is no negative impact from this problem since both copies of that class are identical (compiled from the exact same source). It is purely a cosmetic issue.
The problem is fixed in Java 9 and in Java 8 update 152.
If it annoys you or affects your apps in any way (it shouldn't), the workaround for IntelliJ IDEA is to disable idea_rt launcher agent by adding idea.no.launcher=true into idea.properties (Help | Edit Custom Properties...). The workaround will take effect on the next restart of the IDE.
I don't recommend disabling IntelliJ IDEA launcher agent, though. It's used for such features as graceful shutdown (Exit button), thread dumps, workarounds a problem with too long command line exceeding OS limits, etc. Losing these features just for the sake of hiding the harmless message is probably not worth it, but it's up to you.
这位外国码友清楚地解释了这个Error的原因，大概意思是说这是Mac上面Java的一个老Bug了，会在那些使用了Java Agent的IDE上运行应用时触发，但这个Error对程序是无影响的，可以无视。在Java 9和Java 1.8.152版本里已经修复了。
解决方案:
点击IJ最上面菜单的Help-Edit Custom Properties，没有这个properties文件的话，IJ会提示创建，然后在里面加上
idea.no.launcher=true
重启IDEA
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