Test source compiled twice

Test that an error message is issued for sources in different directories
but with the same filename and package.

Author: odersky

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -165,7 +165,11 @@ class CompilationTests extends ParallelTesting {
     compileFile("tests/neg-custom-args/i3627.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/matchtype-loop.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/completeFromSource/nested/Test1.scala", defaultOptions.and("-sourcepath", "tests/neg-custom-args", "-scansource")) +
-    compileFile("tests/neg-custom-args/repeatedArgs213.scala", defaultOptions.and("-Ynew-collections"))
+    compileFile("tests/neg-custom-args/repeatedArgs213.scala", defaultOptions.and("-Ynew-collections")) +
+    compileList("duplicate source", List(
+      "tests/neg-custom-args/toplevel-samesource/S.scala",
+      "tests/neg-custom-args/toplevel-samesource/nested/S.scala"),
+      defaultOptions)
   }.checkExpectedErrors()
 
   @Test def fuzzyAll: Unit = {

tests/neg-custom-args/toplevel-samesource/S.scala

@@ -0,0 +1,2 @@
+package p
+val a = 1

tests/neg-custom-args/toplevel-samesource/nested/S.scala

@@ -0,0 +1,2 @@
+package p
+val b = 2 // error: package object S$package in package p has already been compiled once during this run