Make `defproc' throw an error if two arguments have the same name.

Related to the already fixed PR 12114 and PR 12133, which motivated the
error, and a few additional typos of the same kind.

(Note that it uses the symbols, but that's how they'll render anyway.)

original commit: 16cd1ad78d6495816a4869621154024652fd8da0

collects/scribble/private/manual-proc.rkt

@@ -238,6 +238,10 @@
       (if (symbol? (car p)) (car p) (loop (car p)))))
   (define (do-one stx-id prototype args arg-contracts arg-vals result-contract
                   first?)
+    (let ([names (remq* '(... ...+) (map arg-id args))])
+      (unless (= (length names) (length (remove-duplicates names eq?)))
+        (error 'defproc "duplicate argument names in prototype for ~s: ~s"
+               (syntax->datum stx-id) names)))
     (define tagged
       (cond
         [(eq? mode 'new)

collects/scribblings/scribble/struct.scrbl

@@ -207,7 +207,7 @@ formats to the current one.}
                                        [alt-path path-string?] [alt-anchor string?]) redirect-target-element?]
 @defproc[(make-link-element [style any/c] [content list?] [tag tag?]) link-element?]
 @defproc[(make-index-element [style any/c] [content list?] [tag tag?]
-                             [tag tag?] [plain-seq (and/c pair? (listof string?))]
+                             [plain-seq (and/c pair? (listof string?))]
                              [entry-seq list?] [desc any/c]) index-element?]
 )]{