Cook doc comments at most once

Doc comments that have already been cooked don't need to be cooked
again, and shouldn't be cooked again, because this would introduce
duplicate symbols.

Author: Duhemm

compiler/src/dotty/tools/dotc/typer/Docstrings.scala

@@ -17,7 +17,7 @@ trait Docstrings { self: Typer =>
     */
   def cookComments(syms: List[Symbol], owner: Symbol)(implicit ctx: Context): Unit =
     ctx.docCtx.foreach { docbase =>
-      val relevantSyms = syms.filter(docbase.docstring(_).isDefined)
+      val relevantSyms = syms.filter(docbase.docstring(_).exists(!_.isExpanded))
       relevantSyms.foreach { sym =>
         expandParentDocs(sym)
         val usecases = docbase.docstring(sym).map(_.usecases).getOrElse(Nil)