Clarify no-duplicates in syntaxes.

preserves.md

@@ -221,7 +221,8 @@ label-`Value` followed by its field-`Value`s.
 curly-brace-enclosed colon-separated pairs of values. `Set`s are
 written either as one or more values enclosed in curly braces, or zero
 or more values enclosed by the tokens `#set{` and
-`}`.[^printing-collections]
+`}`.[^printing-collections] It is an error for a set to contain
+duplicate elements or for a dictionary to contain duplicate keys.
 
           Sequence = "[" *Value ws "]"
         Dictionary = "{" *(Value ws ":" Value) ws "}"
@@ -596,16 +597,16 @@ are two `Value`s in each key-value pair.
 Format C (streaming):
 
             [[ [X_1...X_m] ]] = open(2,1) ++ [[X_1]] ++...++ [[X_m]] ++ close()
-        [[ #set{X_1...X_m} ]] = open(2,2) ++ [[X_1]] ++...++ [[X_m]] ++ close()
+        [[ #set{E_1...E_m} ]] = open(2,2) ++ [[E_1]] ++...++ [[E_m]] ++ close()
     [[ {K_1:V_1...K_m:V_m} ]] = open(2,3) ++ [[K_1]] ++ [[V_1]] ++...
                                           ++ [[K_m]] ++ [[V_m]] ++ close()
 
 Applications may use whichever format suits their needs on a
 case-by-case basis.
 
-There is *no* ordering requirement on the `X_i` elements or
+There is *no* ordering requirement on the `E_i` elements or
 `K_i`/`V_i` pairs.[^no-sorting-rationale] They may appear in any
-order.
+order. However, the `E_i` and `K_i` must be pairwise distinct.
 
   [^no-sorting-rationale]: In the BitTorrent encoding format,
     [bencoding](http://www.bittorrent.org/beps/bep_0003.html#bencoding),