Android error execution failed for task: app:dexDebug

Solution 1:
The right answer is, that some of your jar files does not compile. You should go into your "build.gradle" file in your project, and look in your dependencies, if your just import some jar files, you could try to remove them, and add one at a time, to see which one of them makes the error.
In my case, i did just that, and when i was importing the last one, the app did compile. So i think, that i perhaps imported to many at a time, and that was the real problem. But now it all works.
Solutions 2:
You must check if the same JAR is being imported again.