diff --git a/config.yml b/config.yml
index 658492a488b..34cd388f485 100644
--- a/config.yml
+++ b/config.yml
@@ -187,6 +187,7 @@ errors:
   - PATTERN_EXPRESSION_AFTER_RANGE
   - PATTERN_EXPRESSION_AFTER_REST
   - PATTERN_HASH_KEY
+  - PATTERN_HASH_KEY_DUPLICATE
   - PATTERN_HASH_KEY_LABEL
   - PATTERN_IDENT_AFTER_HROCKET
   - PATTERN_LABEL_AFTER_COMMA
diff --git a/src/prism.c b/src/prism.c
index 6d46e06c974..15c7e4568d5 100644
--- a/src/prism.c
+++ b/src/prism.c
@@ -14828,12 +14828,20 @@ parse_pattern_hash_implicit_value(pm_parser_t *parser, pm_constant_id_list_t *ca
     return (pm_node_t *) pm_implicit_node_create(parser, (pm_node_t *) target);
 }
 
+static void
+parse_pattern_hash_key(pm_parser_t *parser, pm_static_literals_t *keys, pm_node_t *node) {
+    if (pm_static_literals_add(parser, keys, node) != NULL) {
+        pm_parser_err_node(parser, node, PM_ERR_PATTERN_HASH_KEY_DUPLICATE);
+    }
+}
+
 /**
  * Parse a hash pattern.
  */
 static pm_hash_pattern_node_t *
 parse_pattern_hash(pm_parser_t *parser, pm_constant_id_list_t *captures, pm_node_t *first_node) {
     pm_node_list_t assocs = { 0 };
+    pm_static_literals_t keys = { 0 };
     pm_node_t *rest = NULL;
 
     switch (PM_NODE_TYPE(first_node)) {
@@ -14843,6 +14851,7 @@ parse_pattern_hash(pm_parser_t *parser, pm_constant_id_list_t *captures, pm_node
             break;
         case PM_SYMBOL_NODE: {
             if (pm_symbol_node_label_p(first_node)) {
+                parse_pattern_hash_key(parser, &keys, first_node);
                 pm_node_t *value;
 
                 if (match7(parser, PM_TOKEN_COMMA, PM_TOKEN_KEYWORD_THEN, PM_TOKEN_BRACE_RIGHT, PM_TOKEN_BRACKET_RIGHT, PM_TOKEN_PARENTHESIS_RIGHT, PM_TOKEN_NEWLINE, PM_TOKEN_SEMICOLON)) {
@@ -14897,6 +14906,8 @@ parse_pattern_hash(pm_parser_t *parser, pm_constant_id_list_t *captures, pm_node
         } else {
             expect1(parser, PM_TOKEN_LABEL, PM_ERR_PATTERN_LABEL_AFTER_COMMA);
             pm_node_t *key = (pm_node_t *) pm_symbol_node_label_create(parser, &parser->previous);
+
+            parse_pattern_hash_key(parser, &keys, key);
             pm_node_t *value = NULL;
 
             if (match7(parser, PM_TOKEN_COMMA, PM_TOKEN_KEYWORD_THEN, PM_TOKEN_BRACE_RIGHT, PM_TOKEN_BRACKET_RIGHT, PM_TOKEN_PARENTHESIS_RIGHT, PM_TOKEN_NEWLINE, PM_TOKEN_SEMICOLON)) {
@@ -14919,6 +14930,7 @@ parse_pattern_hash(pm_parser_t *parser, pm_constant_id_list_t *captures, pm_node
     pm_hash_pattern_node_t *node = pm_hash_pattern_node_node_list_create(parser, &assocs, rest);
     xfree(assocs.nodes);
 
+    pm_static_literals_free(&keys);
     return node;
 }
 
diff --git a/templates/src/diagnostic.c.erb b/templates/src/diagnostic.c.erb
index b403eeaa8c8..301f98134f5 100644
--- a/templates/src/diagnostic.c.erb
+++ b/templates/src/diagnostic.c.erb
@@ -270,6 +270,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_PATTERN_EXPRESSION_AFTER_RANGE]     = { "expected a pattern expression after the range operator", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_PATTERN_EXPRESSION_AFTER_REST]      = { "unexpected pattern expression after the `**` expression", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_PATTERN_HASH_KEY]                   = { "expected a key in the hash pattern", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_PATTERN_HASH_KEY_DUPLICATE]         = { "duplicated key name", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_PATTERN_HASH_KEY_LABEL]             = { "expected a label as the key in the hash pattern", PM_ERROR_LEVEL_SYNTAX }, // TODO // THIS // AND // ABOVE // IS WEIRD
     [PM_ERR_PATTERN_IDENT_AFTER_HROCKET]        = { "expected an identifier after the `=>` operator", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_PATTERN_LABEL_AFTER_COMMA]          = { "expected a label after the `,` in the hash pattern", PM_ERROR_LEVEL_SYNTAX },
diff --git a/test/prism/errors_test.rb b/test/prism/errors_test.rb
index b3ae1c8ec59..d9dedc88e1e 100644
--- a/test/prism/errors_test.rb
+++ b/test/prism/errors_test.rb
@@ -2161,8 +2161,7 @@ def test_duplicate_pattern_capture
       source = <<~RUBY
         () => [a, a]
         () => [a, *a]
-        () => {a:, a:}
-        () => {a: a, a: a}
+        () => {a: a, b: a}
         () => {a: a, **a}
         () => [a, {a:}]
         () => [a, {a: {a: {a: [a]}}}]
@@ -2173,6 +2172,12 @@ def test_duplicate_pattern_capture
       assert_error_messages source, Array.new(source.lines.length, "duplicated variable name"), compare_ripper: false
     end
 
+    def test_duplicate_pattern_hash_key
+      assert_error_messages "() => {a:, a:}", ["duplicated key name", "duplicated variable name"]
+      assert_error_messages "() => {a:1, a:2}", ["duplicated key name"]
+      refute_error_messages "() => [{a:1}, {a:2}]"
+    end
+
     private
 
     def assert_errors(expected, source, errors, compare_ripper: RUBY_ENGINE == "ruby")
@@ -2192,6 +2197,11 @@ def assert_error_messages(source, errors, compare_ripper: RUBY_ENGINE == "ruby")
       assert_equal(errors, result.errors.map(&:message))
     end
 
+    def refute_error_messages(source, compare_ripper: RUBY_ENGINE == "ruby")
+      refute_nil Ripper.sexp_raw(source) if compare_ripper
+      assert Prism.parse_success?(source)
+    end
+
     def assert_warning_messages(source, warnings)
       result = Prism.parse(source)
       assert_equal(warnings, result.warnings.map(&:message))