Validate that set elements are unique during parse

src/preserves/private/parse.nim

@@ -66,6 +66,7 @@ proc parsePreserves*(text: string): Preserve[void] {.gcsafe.} =
       var prs = Value(kind: pkSet)
       for frame in stack.mitems:
         if frame.pos > capture[0].si:
+          for e in prs.set: validate(e != frame.value)
           prs.incl(move frame.value)
       stack.shrink prs.set.len
       pushStack prs