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.

Since “this message is harmless”(see the @CrazyCoder's answer), a simple and safe workaround is that you can fold this buzzing message in console by IntelliJ IDEA settings:

1. 【Preferences】- 【Editor】-【General】-【Console】- 【Fold console lines that contain】
   Of course, you can use 【Find Action...】(cmd+shift+A on mac) and type Fold console lines that contain so as to navigate more effectively.
2. add Class JavaLaunchHelper is implemented in both

On my computer, It turns out: (LGTM :b )

And you can unfold the message to check it again:

PS:

As of October 2017, this issue is now resolved in jdk1.9/jdk1.8.152/jdk1.7.161
for more info, see the @muttonUp's answer)

I am using Intellij Idea 2017 and I got into the same problem. What solved the problem for me was to simply

1. close the project in intelliJ
2. File -> New -> project from existing resources
3. use Import from external model (if any)
4. open the project again.