document and test dict behavior on bad association lists

Follows up on ce3992dbf3 to document and test the constraints
that required reverting the earlier change.

pkgs/racket-doc/scribblings/reference/dicts.scrbl

@@ -15,14 +15,20 @@ values. The following datatypes are all dictionaries:
 
  @item{@techlink{vectors} (using only exact integers as keys);}
 
- @item{@techlink{lists} of @techlink{pairs} (an @deftech{association
-       list} using @racket[equal?] to compare keys); and}
+ @item{@techlink{lists} of @techlink{pairs} as an @deftech{association
+       list} using @racket[equal?] to compare keys, which must be distinct; and}
 
  @item{@techlink{structures} whose types implement the @racket[gen:dict]
        @tech{generic interface}.}
 
 ]
 
+When list of pairs is used as @tech{association list} but does not
+have distinct keys (so it's not an association list), operations like
+@racket[dict-ref] and @racket[dict-remove] operate on the first
+instance of the key, while operations like @racket[dict-map] and
+@racket[dict-keys] produce an element for every instance of the key.
+
 @note-lib[racket/dict]
 
 @section{Dictionary Predicates and Contracts}

pkgs/racket-test-core/tests/racket/dict.rktl

@@ -163,6 +163,16 @@
     ;; preserve from GC:
     (list s1 s2)))
 
+;; Check behavior on a list of pairs that isn't
+;; a dictionary due to duplicate keys:
+(test 1 dict-ref '((a . 1) (b . 2) (a . 3)) 'a)
+(test '((b . 2) (a . 3)) dict-remove '((a . 1) (b . 2) (a . 3)) 'a)
+(test '((a . 4) (b . 2) (a . 3)) dict-set '((a . 1) (b . 2) (a . 3)) 'a 4)
+(test 3 dict-count '((a . 1) (b . 2) (a . 3)))
+(test '((a 1) (b 2) (a 3)) dict-map '((a . 1) (b . 2) (a . 3)) list)
+(test '(a b a) dict-keys '((a . 1) (b . 2) (a . 3)))
+(test '(1 2 3) dict-values '((a . 1) (b . 2) (a . 3)))
+
 ;; ----------------------------------------
 
 (report-errs)