fix: CREATE ENUMERATION rejects duplicate value names (#390)

ValidateEnumeration only checked for reserved words; it never detected
duplicate value names within the same CREATE ENUMERATION statement.
Running CREATE ENUMERATION M.E (A 'First', A 'Second') succeeded silently,
producing an invalid model that Studio Pro flags as an error.

Added a seen-name set (case-insensitive) to ValidateEnumeration.
A second occurrence of a name emits a MDL011 violation, which the executor
turns into a validation error before any write reaches the backend.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ako

mdl/executor/cmd_enumerations.go

@@ -289,8 +289,24 @@ var mendixReservedWords = map[string]bool{
 // This function does not require a project connection.
 func ValidateEnumeration(stmt *ast.CreateEnumerationStmt) []linter.Violation {
 	var violations []linter.Violation
+	seen := make(map[string]bool)
 	for _, v := range stmt.Values {
-		if mendixReservedWords[strings.ToLower(v.Name)] {
+		lower := strings.ToLower(v.Name)
+		if seen[lower] {
+			violations = append(violations, linter.Violation{
+				RuleID:   "MDL011",
+				Severity: linter.SeverityError,
+				Message: fmt.Sprintf(
+					"duplicate enumeration value name '%s'", v.Name),
+				Location: linter.Location{
+					DocumentType: "enumeration",
+					DocumentName: stmt.Name.String(),
+				},
+				Suggestion: "Each enumeration value must have a unique name",
+			})
+		}
+		seen[lower] = true
+		if mendixReservedWords[lower] {
 			violations = append(violations, linter.Violation{
 				RuleID:   "MDL010",
 				Severity: linter.SeverityError,

mdl/executor/cmd_enumerations_mock_test.go

@@ -155,5 +155,25 @@ func TestDropEnumeration_Qualified_Success(t *testing.T) {
 	}
 }
 
+// Issue #390 — CREATE ENUMERATION must reject duplicate value names.
+func TestCreateEnumeration_DuplicateValueName_Issue390(t *testing.T) {
+	mod := mkModule("M")
+	mb := &mock.MockBackend{
+		IsConnectedFunc:      func() bool { return true },
+		ListModulesFunc:      func() ([]*model.Module, error) { return []*model.Module{mod}, nil },
+		ListEnumerationsFunc: func() ([]*model.Enumeration, error) { return nil, nil },
+	}
+	ctx, _ := newMockCtx(t, withBackend(mb))
+	err := execCreateEnumeration(ctx, &ast.CreateEnumerationStmt{
+		Name: ast.QualifiedName{Module: "M", Name: "E"},
+		Values: []ast.EnumValue{
+			{Name: "A", Caption: "First"},
+			{Name: "A", Caption: "Second"},
+		},
+	})
+	assertError(t, err)
+	assertContainsStr(t, err.Error(), "duplicate")
+}
+
 // Backend error: cmd_error_mock_test.go (TestShowEnumerations_Mock_BackendError)
 // JSON: cmd_json_mock_test.go (TestShowEnumerations_Mock_JSON)