Error on duplicate keys

internal/topdata/topdata.go

@@ -264,6 +264,7 @@ func ValidateProject(p *project.Project) ValidationReport {
 	})
 	validateTopPackageAssets(sourceDir, dataDir, &report)
 	validateNativeOutputCatalog(dataDir, &report)
+	validateNativeEntryKeyUniqueness(dataDir, &report)
 	validateNativeLockAllocation(dataDir, &report)
 	validateGeneratedFeatFamilies(dataDir, &report)
 	validateItempropsRegistryGraph(dataDir, &report)
@@ -1001,6 +1002,64 @@ func validateNativeOutputCatalog(dataDir string, report *ValidationReport) {
 	}
 }
 
+func validateNativeEntryKeyUniqueness(dataDir string, report *ValidationReport) {
+	datasets, err := discoverNativeDatasets(dataDir)
+	if err != nil {
+		report.Diagnostics = append(report.Diagnostics, Diagnostic{
+			Severity: SeverityError,
+			Path:     dataDir,
+			Message:  fmt.Sprintf("discover native datasets for entry key uniqueness validation: %v", err),
+		})
+		return
+	}
+	for _, dataset := range datasets {
+		if dataset.Kind != nativeDatasetBase {
+			continue
+		}
+		entryKeyPath := map[string]string{}
+		for _, dir := range []string{dataset.ModulesDir, dataset.GeneratedDir} {
+			paths, err := collectModulePaths(dir)
+			if err != nil {
+				report.Diagnostics = append(report.Diagnostics, Diagnostic{
+					Severity: SeverityError,
+					Path:     dir,
+					Message:  fmt.Sprintf("collect module paths for entry key uniqueness validation: %v", err),
+				})
+				continue
+			}
+			for _, path := range paths {
+				obj, err := loadJSONObject(path)
+				if err != nil {
+					report.Diagnostics = append(report.Diagnostics, Diagnostic{
+						Severity: SeverityError,
+						Path:     path,
+						Message:  fmt.Sprintf("load module for entry key uniqueness validation: %v", err),
+					})
+					continue
+				}
+				entries, ok := obj["entries"].(map[string]any)
+				if !ok {
+					continue
+				}
+				for _, key := range sortedKeys(entries) {
+					if key == "" {
+						continue
+					}
+					if previousPath, exists := entryKeyPath[key]; exists {
+						report.Diagnostics = append(report.Diagnostics, Diagnostic{
+							Severity: SeverityError,
+							Path:     path,
+							Message:  fmt.Sprintf("duplicate entries key %q declared by %s and %s", key, previousPath, path),
+						})
+						continue
+					}
+					entryKeyPath[key] = path
+				}
+			}
+		}
+	}
+}
+
 func validateNativeLockAllocation(dataDir string, report *ValidationReport) {
 	datasets, err := discoverNativeDatasets(dataDir)
 	if err != nil {

internal/topdata/topdata_test.go

@@ -134,6 +134,39 @@ func TestValidateProjectRejectsDuplicateJSONKeys(t *testing.T) {
 	}
 }
 
+func TestValidateProjectRejectsDuplicateEntryKeysAcrossModules(t *testing.T) {
+	root := testProjectRoot(t)
+	mkdirAll(t, filepath.Join(root, "topdata", "data", "baseitems", "modules"))
+	writeFile(t, filepath.Join(root, "topdata", "base_dialog.json"), "{}\n")
+	writeFile(t, filepath.Join(root, "topdata", "data", "baseitems", "base.json"), `{
+  "output": "baseitems.2da",
+  "columns": ["Label", "MaxRange"],
+  "rows": []
+}`+"\n")
+	writeFile(t, filepath.Join(root, "topdata", "data", "baseitems", "lock.json"), "{}\n")
+	writeFile(t, filepath.Join(root, "topdata", "data", "baseitems", "modules", "10_add_sign.json"), `{
+  "entries": {
+    "baseitems:holdable_sign": {"Label": "Holdable Sign", "MaxRange": "100"}
+  }
+}`+"\n")
+	writeFile(t, filepath.Join(root, "topdata", "data", "baseitems", "modules", "20_add_sign_again.json"), `{
+  "entries": {
+    "baseitems:holdable_sign": {"Label": "Duplicate Sign", "MaxRange": "200"}
+  }
+}`+"\n")
+
+	report := ValidateProject(testProject(root))
+	if !report.HasErrors() {
+		t.Fatal("expected duplicate entry key validation error")
+	}
+	text := diagnosticsText(report.Diagnostics)
+	if !strings.Contains(text, `duplicate entries key "baseitems:holdable_sign"`) ||
+		!strings.Contains(text, "10_add_sign.json") ||
+		!strings.Contains(text, "20_add_sign_again.json") {
+		t.Fatalf("expected duplicate entry key diagnostic with both module paths, got:\n%s", text)
+	}
+}
+
 func TestResolvedTableRegistryRegistersConsistentTables(t *testing.T) {
 	collected := []nativeCollectedDataset{
 		{