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There is only one for play as well

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.

That is correct. There are 2 separate APIs (that come from a single project). This is because Scala can support a bunch of functionality that Java (and Kotlin) can't support (type classes, higher kinded types, etc.). So your options are to hamstring the API for Scala or have 2 separate APIs.

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