Re-add error for duplicate keys in map patterns. (#2969)

Author: munificent

accepted/future-releases/0546-patterns/feature-specification.md

@@ -4,7 +4,7 @@ Author: Bob Nystrom
 
 Status: Accepted
 
-Version 2.29 (see [CHANGELOG](#CHANGELOG) at end)
+Version 2.30 (see [CHANGELOG](#CHANGELOG) at end)
 
 Note: This proposal is broken into a couple of separate documents. See also
 [records][] and [exhaustiveness][].
@@ -621,6 +621,15 @@ It is a compile-time error if:
     expressions in a future release without it being a breaking change, similar
     to default values in parameter lists.*
 
+*   Any two keys in the map are identical. *Duplicate keys are likely to be a
+    copy/paste error. If you want to match the value associated with some key
+    against multiple patterns, you can always use an `||` pattern.*s
+
+*   Any two record keys which both have primitive equality are equal. *Since
+    records don't have defined identity, we can't use the previous rule to
+    detect identical records. But records do support an equality test known at
+    compile time if all of their fields do, so we use that.*
+
 *Note that `mapPatternEntries` is not optional, which means it is an error for
 a map pattern to be empty.*
 
@@ -3501,6 +3510,10 @@ Here is one way it could be broken down into separate pieces:
 
 ## Changelog
 
+### 2.30
+
+-   Re-add error for map patterns to have duplicate keys.
+
 ### 2.29
 
 -   Map patterns no longer check length.