Play has two actor systems when run in “dev mode” (using sbt’s run command). The main actor system picks up settings from configuration under the akka namespace. The dev mode server actor system (which you probably don’t want in the cluster) picks up settings from play.akka.dev-mode first and then akka as a fallback. What you could try doing is putting all your cluster settings under akka - like you’re doing now - but then also add settings to disable the cluster under play.akka.dev-mode. This should result in the clustering running for one actor system but not both.
See this thread for more info: Configuring Akka HTTP backend