in extractor-legacy-0.13/src/main/scala/org/jetbrains/sbt/extractors/ProjectExtractor.scala [247:258]
private def mergeConfigurations(
configurations: Seq[ConfigurationData]
): Seq[ConfigurationData] =
configurations
.groupBy(_.id)
.map {
case (id, confs) =>
val sources = confs.flatMap(_.sources).distinct
val resources = confs.flatMap(_.resources).distinct
val excludes = confs.flatMap(_.excludes).distinct
ConfigurationData(id, sources, resources, excludes, confs.head.classes)
}