dedup unmanagedSourceDirectories for Scala 3

For Scala 3, scalaBinaryVersion == epochVersion

main/src/main/scala/sbt/Defaults.scala

@@ -745,7 +745,7 @@ object Defaults extends BuildCommon {
         scalaSrcDir.getParentFile / s"${scalaSrcDir.name}-$sv",
         scalaSrcDir.getParentFile / s"${scalaSrcDir.name}-$epochVersion",
         javaSrcDir,
-      )
+      ).distinct
     else
       Seq(scalaSrcDir, javaSrcDir)
   }

sbt-app/src/sbt-test/project/scala3-binary-version/build.sbt

@@ -0,0 +1,11 @@
+ThisBuild / scalaVersion := "3.0.0"
+
+lazy val check = taskKey[Unit]("check")
+
+lazy val root = project.in(file("."))
+  .settings(
+    check := {
+      val dirs = (Compile / unmanagedSourceDirectories).value
+      assert(dirs == dirs.distinct)
+    }
+  )

sbt-app/src/sbt-test/project/scala3-binary-version/test

@@ -0,0 +1 @@
+> check