diff --git a/pkg/analysis_server/test/tool/lsp_spec/codegen_test.dart b/pkg/analysis_server/test/tool/lsp_spec/codegen_test.dart
index fbaa2cb3af7..b6b5a490ca0 100644
--- a/pkg/analysis_server/test/tool/lsp_spec/codegen_test.dart
+++ b/pkg/analysis_server/test/tool/lsp_spec/codegen_test.dart
@@ -22,7 +22,7 @@ class CodegenTest {
       LspEnum(
         name: 'x',
         typeOfValues: TypeReference.int,
-        members: [
+        constants: [
           Constant(name: 'new', type: TypeReference.string, value: '1'),
         ],
       ),
diff --git a/pkg/analysis_server/test/tool/lsp_spec/dart_test.dart b/pkg/analysis_server/test/tool/lsp_spec/dart_test.dart
index 63bd35ed7c9..8c6336f1531 100644
--- a/pkg/analysis_server/test/tool/lsp_spec/dart_test.dart
+++ b/pkg/analysis_server/test/tool/lsp_spec/dart_test.dart
@@ -29,13 +29,13 @@ ast.LspEnum _enum({
   String name = 'x',
   String type = 'int',
   bool flags = false,
-  required List<ast.Member> members,
+  required List<ast.Constant> constants,
 }) {
   return ast.LspEnum(
     name: name,
     typeOfValues: ast.TypeReference(type),
     flags: flags,
-    members: members,
+    constants: constants,
   );
 }
 
@@ -48,22 +48,35 @@ ast.UnionType _union(List<String> names) =>
 class DartTest {
   void test_flags_requiresPowerOfTwo_double() {
     expect(
-      () => _enum(flags: true, members: [_constant(1.2)]),
-      throwsA(isA<AssertionError>()),
+      () => _enum(flags: true, constants: [_constant(1.2)]),
+      throwsArgumentError,
     );
   }
 
   void test_flags_requiresPowerOfTwo_nonPowerOfTwo() {
     expect(
-      () => _enum(flags: true, members: [_constant(3)]),
-      throwsA(isA<AssertionError>()),
+      () => _enum(flags: true, constants: [_constant(3)]),
+      throwsArgumentError,
     );
   }
 
   void test_flags_requiresPowerOfTwo_string() {
     expect(
-      () => _enum(flags: true, members: [_constant('test')]),
-      throwsA(isA<AssertionError>()),
+      () => _enum(flags: true, constants: [_constant('test')]),
+      throwsArgumentError,
+    );
+  }
+
+  void test_flags_requiresUniqueValues() {
+    expect(
+      () => _enum(
+        flags: true,
+        constants: [
+          _constant(1, name: 'a'),
+          _constant(1, name: 'b'),
+        ],
+      ),
+      throwsArgumentError,
     );
   }
 
diff --git a/pkg/analysis_server/test/tool/lsp_spec/meta_model_test.dart b/pkg/analysis_server/test/tool/lsp_spec/meta_model_test.dart
index a161958ed86..9b7842928c9 100644
--- a/pkg/analysis_server/test/tool/lsp_spec/meta_model_test.dart
+++ b/pkg/analysis_server/test/tool/lsp_spec/meta_model_test.dart
@@ -385,13 +385,10 @@ Sometimes after a blank line we'll have a note.
 
     expect(output[0], const TypeMatcher<LspEnum>());
     var namespace = output[0] as LspEnum;
-    expect(namespace.members, hasLength(3));
-    for (var m in namespace.members) {
-      expect(m, const TypeMatcher<Constant>());
-    }
-    var create = namespace.members[0] as Constant,
-        delete = namespace.members[1] as Constant,
-        rename = namespace.members[2] as Constant;
+    expect(namespace.constants, hasLength(3));
+    var create = namespace.constants[0],
+        delete = namespace.constants[1],
+        rename = namespace.constants[2];
     expect(create.name, equals('Create'));
     expect(create.type, isSimpleType('ResourceOperationKind'));
     expect(create.comment, equals('Supports creating new files and folders.'));
diff --git a/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart b/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
index c3ff4b3efa6..9c6183cfc23 100644
--- a/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
+++ b/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
@@ -633,7 +633,7 @@ void _writeDocCommentsAndAnnotations(
 
 void _writeEnumClass(IndentableStringBuffer buffer, LspEnum namespace) {
   _writeDocCommentsAndAnnotations(buffer, namespace);
-  var consts = namespace.members.cast<Constant>().toList();
+  var consts = namespace.constants;
   var namespaceName = namespace.name;
   var typeOfValues = namespace.typeOfValues;
   var allowsAnyValue = enumClassAllowsAnyValue(namespaceName);
@@ -674,10 +674,10 @@ void _writeEnumClass(IndentableStringBuffer buffer, LspEnum namespace) {
       ..outdent()
       ..writeIndentedln('}');
   }
-  namespace.members.whereType<Constant>().forEach((cons) {
+  for (var cons in consts) {
     // We don't use any deprecated enum values, so omit them entirely.
     if (cons.isDeprecated) {
-      return;
+      continue;
     }
     _writeDocCommentsAndAnnotations(buffer, cons);
     var memberName = cons.dartSafeName;
@@ -685,7 +685,7 @@ void _writeEnumClass(IndentableStringBuffer buffer, LspEnum namespace) {
     buffer.writeIndentedln(
       'static const $memberName = $namespaceName$constructorName($value);',
     );
-  });
+  }
   if (namespace.flags) {
     buffer
       ..writeln()
diff --git a/pkg/analysis_server/tool/lsp_spec/custom/interactive_forms.dart b/pkg/analysis_server/tool/lsp_spec/custom/interactive_forms.dart
index 6a2502f4b66..5292c563cdc 100644
--- a/pkg/analysis_server/tool/lsp_spec/custom/interactive_forms.dart
+++ b/pkg/analysis_server/tool/lsp_spec/custom/interactive_forms.dart
@@ -137,7 +137,7 @@ final interactiveFormClasses = <LspEntity>[
     name: 'FileExistence',
     typeOfValues: TypeReference.int,
     flags: true,
-    members: [
+    constants: [
       // Values should be powers of 2 to allow New|Existing.
       Constant(
         name: 'New',
@@ -161,7 +161,7 @@ final interactiveFormClasses = <LspEntity>[
     name: 'FileType',
     typeOfValues: TypeReference.int,
     flags: true,
-    members: [
+    constants: [
       // Values should be powers of 2 to allow Regular|Directory.
       Constant(
         name: 'Regular',
diff --git a/pkg/analysis_server/tool/lsp_spec/meta_model.dart b/pkg/analysis_server/tool/lsp_spec/meta_model.dart
index dfca1dbb254..ffed2163dfc 100644
--- a/pkg/analysis_server/tool/lsp_spec/meta_model.dart
+++ b/pkg/analysis_server/tool/lsp_spec/meta_model.dart
@@ -185,23 +185,49 @@ abstract class LspEntity {
 class LspEnum extends LspEntity {
   final TypeBase typeOfValues;
   final bool flags;
-  final List<Member> members;
+  final List<Constant> constants;
   LspEnum({
     required super.name,
     super.comment,
     super.isProposed,
     required this.typeOfValues,
     this.flags = false,
-    required this.members,
-  }) : assert(
-         !flags ||
-             members
-                 .whereType<Constant>()
-                 .map((member) => int.tryParse(member.value))
-                 .every((value) => value != null && _isPowerOfTwo(value)),
-         'flags enums require all enum values to be int powers of two.',
-       ) {
-    members.sortBy((member) => member.name.toLowerCase());
+    required this.constants,
+  }) {
+    _validateFlagsEnum();
+    constants.sortBy((constant) => constant.name.toLowerCase());
+  }
+
+  void _validateFlagsEnum() {
+    if (!flags) {
+      return;
+    }
+
+    if (typeOfValues.dartTypeWithTypeArgs != 'int') {
+      throw ArgumentError(
+        'Flags enum $name has value type ${typeOfValues.dartTypeWithTypeArgs}; '
+        'flags enum values must be ints.',
+      );
+    }
+
+    var usedValues = <int>{};
+    for (var constant in constants) {
+      var value = int.tryParse(constant.value);
+      if (value == null || !_isPowerOfTwo(value)) {
+        throw ArgumentError(
+          'Flags enum $name constant ${constant.name} has value '
+          '${constant.value}; '
+          'flags enum values must be int powers of two.',
+        );
+      }
+
+      if (!usedValues.add(value)) {
+        throw ArgumentError(
+          'Flags enum $name has duplicate value $value; '
+          'flags enum values must be unique.',
+        );
+      }
+    }
   }
 }
 
diff --git a/pkg/analysis_server/tool/lsp_spec/meta_model_cleaner.dart b/pkg/analysis_server/tool/lsp_spec/meta_model_cleaner.dart
index 84cbbdf8bf3..534337184ed 100644
--- a/pkg/analysis_server/tool/lsp_spec/meta_model_cleaner.dart
+++ b/pkg/analysis_server/tool/lsp_spec/meta_model_cleaner.dart
@@ -191,9 +191,9 @@ class LspMetaModelCleaner {
       isProposed: namespace.isProposed,
       typeOfValues: namespace.typeOfValues,
       flags: namespace.flags,
-      members: namespace.members
+      constants: namespace.constants
           .where(_includeEntityInOutput)
-          .map((member) => _cleanMember(namespace.name, member))
+          .map(_cleanConst)
           .toList(),
     );
   }
@@ -387,7 +387,7 @@ class LspMetaModelCleaner {
         isProposed: dest.isProposed,
         typeOfValues: dest.typeOfValues,
         flags: dest.flags || source.flags,
-        members: [...dest.members, ...source.members],
+        constants: [...dest.constants, ...source.constants],
       );
     } else if (source is Interface && dest is Interface) {
       return Interface(
@@ -533,7 +533,7 @@ class LspMetaModelCleaner {
           isProposed: type.isProposed,
           typeOfValues: type.typeOfValues,
           flags: type.flags,
-          members: type.members,
+          constants: type.constants,
         );
       } else {
         throw 'Renaming ${type.runtimeType} is not implemented';
diff --git a/pkg/analysis_server/tool/lsp_spec/meta_model_reader.dart b/pkg/analysis_server/tool/lsp_spec/meta_model_reader.dart
index 36803ad17ea..94f8669aa93 100644
--- a/pkg/analysis_server/tool/lsp_spec/meta_model_reader.dart
+++ b/pkg/analysis_server/tool/lsp_spec/meta_model_reader.dart
@@ -60,7 +60,7 @@ class LspMetaModelReader {
 
     return LspMetaModel(
       types: types,
-      methods: methodsEnum?.members.cast<Constant>().toList() ?? [],
+      methods: methodsEnum?.constants.toList() ?? [],
     );
   }
 
@@ -106,7 +106,7 @@ class LspMetaModelReader {
       name: 'Method',
       comment: comment,
       typeOfValues: TypeReference.string,
-      members: methodConstants,
+      constants: methodConstants,
     );
   }
 
@@ -299,7 +299,7 @@ class LspMetaModelReader {
       comment: documentation,
       isProposed: _isProposed(documentation),
       typeOfValues: baseType,
-      members: [
+      constants: [
         ...?(model['values'] as List?)?.map((p) => _extractEnumValue(type, p)),
       ],
     );