Fix spurious warnings in TreeChecker.

Author: DarkDimius

src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -50,8 +50,7 @@ class TreeChecker extends Phase with SymTransformer {
 
   def testDuplicate(sym: Symbol, registry: mutable.Map[String, Symbol], typ: String)(implicit ctx: Context) = {
     val name = sym.fullName.toString
-    if (registry.contains(name))
-      if (this.flatClasses || !(sym.isAnonymousFunction || sym.isAnonymousClass || sym.isAnonymousModuleVal))
+    if (this.flatClasses && registry.contains(name))
         printError(s"$typ defined twice $sym ${sym.id} ${registry(name).id}")
     registry(name) = sym
   }