Okay, poorly worded. I think (please correct me if I'm wrong) that in play you have distinct parts of the code-base that are for Java developers, while other parts are for Scala developers.

In Javalin, although some parts are written in Kotlin and some parts are written in Java, there is 0 duplication. If one part is written in Kotlin, both Java and Kotlin developers use that part. If a part is written in Java, both Java and Kotlin developers use that part.