This drove me nuts once upon a time. Java7 apparently comes with 2 jars that include different implementations of this same class. jfxrt.jar and plugin.jar
I solved issues with these by simply removing jfxrt.jar from my classpath. You'll have to dig for how to do that for your build system. In Intellij, you can go to:
File -> Project Structure -> SDKs
Then, on the classpath tab, highlight jfxrt.jar and click '-'
ETA: I found the answer that originally helped me that has a bit more info: https://stackoverflow.com/a/14156602/1057157