[Bug #22314] Check **rest in patterns for duplicates and alternations

`p_kwrest` was the only binding rule in patterns that skipped
`error_duplicate_pattern_variable`, so parse.y accepted `{ a: x, **x }`
and `1 | { a: 1, **rest }` at parse time while Prism rejects both. Route
it through the same check as `*rest`.

parse.y

@@ -5749,6 +5749,7 @@ p_kw_label	: tLABEL
 
 p_kwrest	: kwrest_mark tIDENTIFIER
                     {
+                        error_duplicate_pattern_variable(p, $2, &@2);
                         $$ = $2;
                     /*% ripper: var_field!($:2) %*/
                     }

test/ruby/test_pattern_matching.rb

@@ -282,6 +282,36 @@ class TestPatternMatching < Test::Unit::TestCase
       end
     }, [], /alternative pattern/,
     success: false)
+
+    assert_in_out_err(['-c'], %q{
+      case 0
+      in { a: 1, **rest } | 1
+      end
+    }, [], /alternative pattern/,
+    success: false)
+
+    assert_in_out_err(['-c'], %q{
+      case 0
+      in 1 | { a: 1, **rest }
+      end
+    }, [], /alternative pattern/,
+    success: false)
+
+    # `**nil` binds nothing
+    assert_valid_syntax(%{
+      case 0
+      in { a: 1, **nil } | 1
+        true
+      end
+    })
+
+    # a `**rest` binding must not leak into an unrelated sibling alternation
+    assert_valid_syntax(%{
+      case 0
+      in [ { **rest }, 1 | 2 ]
+        true
+      end
+    })
   end
 
   def test_alternative_pattern_nested
@@ -372,6 +402,18 @@ class TestPatternMatching < Test::Unit::TestCase
       end
     }, /duplicated variable name/)
 
+    assert_syntax_error(%q{
+      case 0
+      in {a: x, **x}
+      end
+    }, /duplicated variable name/)
+
+    assert_syntax_error(%q{
+      case 0
+      in {a:, **a}
+      end
+    }, /duplicated variable name/)
+
     assert_block do
       case [0, "1"]
       in a, "#{case 1; in a; a; end}"