Add duplicate settings ID detection for theme checker

Implements a new validation check to prevent duplicate setting IDs
across theme configurations. This helps theme developers identify and
fix duplicate IDs early in development, preventing potential conflicts
and overrides.

The implementation includes:
- New JSONCheckDefinition for unique setting ID validation
- Test coverage for valid and invalid theme configurations
- Detection logic for finding duplicate IDs in settings schema

See:
Ticket: 49020903
Shopify/cli#4187

packages/theme-check-common/src/checks/unique-settings-id/index.spec.ts

@@ -0,0 +1,24 @@
+import { expect, describe, it } from 'vitest';
+import { UniqueSettingIds } from './index';
+import { highlightedOffenses, runJSONCheck } from '../../test';
+import { invalidJson, validJson } from './test-data';
+
+describe('Module: UniqueSettingIds', () => {
+  it("Should report an error for duplicate id's in settings_schema (0)", async () => {
+    const offenses = await runJSONCheck(UniqueSettingIds, invalidJson, 'file.json');
+
+    expect(offenses).to.have.length(1);
+    expect(offenses[0].message).to.equal('Duplicate setting id found: "nosto_account_id"');
+
+    const highlights = highlightedOffenses({ 'file.json': invalidJson }, offenses);
+
+    expect(highlights).to.have.length(1);
+    expect(highlights[0]).to.equal('"id": "nosto_account_id"');
+  });
+
+  it('should not report any errors for valid file', async () => {
+    const offenses = await runJSONCheck(UniqueSettingIds, validJson);
+
+    expect(offenses).to.be.empty;
+  });
+});

packages/theme-check-common/src/checks/unique-settings-id/index.ts

@@ -0,0 +1,84 @@
+import {
+  isArrayNode,
+  isLiteralNode,
+  isObjectNode,
+  isPropertyNode,
+  Severity,
+  SourceCodeType,
+} from '../../types';
+
+import type { ArrayNode, PropertyNode, JSONCheckDefinition } from '../../types';
+
+export const UniqueSettingIds: JSONCheckDefinition = {
+  meta: {
+    code: 'UniqueSettingId',
+    name: 'Prevent duplicate Ids in setting_schema',
+    docs: {
+      description: 'This check is aimed at eliminating duplicate Ids in settings_schema.json',
+      recommended: true,
+      // url: 'https://shopify.dev/docs/storefronts/themes/tools/theme-check/checks/valid-schema',
+    },
+    type: SourceCodeType.JSON,
+    severity: Severity.ERROR,
+    schema: {},
+    targets: [],
+  },
+
+  create(context) {
+    return {
+      async onCodePathEnd(file) {
+        if (isArrayNode(file.ast)) {
+          const settingIds: PropertyNode[] = [];
+
+          /* Find and loop through all of our nodes that have an id value and find their key value */
+          for (const child of file.ast.children) {
+            if (isObjectNode(child) && child.children) {
+              const settingsNode = child.children.find((node) => node.key.value === 'settings');
+
+              if (settingsNode && settingsNode.value && isArrayNode(settingsNode.value)) {
+                for (const setting of settingsNode.value.children) {
+                  if (isObjectNode(setting) && setting.children) {
+                    const idNode = setting.children.find((node) => node.key.value === 'id');
+                    if (isPropertyNode(idNode)) {
+                      settingIds.push(idNode);
+                    }
+                  }
+                }
+              }
+            }
+          }
+
+          /* Check for dupes */
+          const idMap = new Map<string, PropertyNode[]>();
+          for (const node of settingIds) {
+            if (isLiteralNode(node.value)) {
+              const id = node.value.value;
+              if (typeof id === 'string') {
+                if (!idMap.has(id)) {
+                  idMap.set(id, []);
+                }
+                idMap.get(id)!.push(node);
+              }
+            }
+          }
+
+          const duplicates: [string, PropertyNode[]][] = Array.from(idMap.entries()).filter(
+            ([_, nodes]) => nodes.length > 1,
+          );
+
+          if (duplicates.length > 0) {
+            for (const [id, nodes] of duplicates) {
+              const lastNodeFound = nodes[nodes.length - 1];
+
+              context.report({
+                message: `Duplicate setting id found: "${id}"`,
+                startIndex: lastNodeFound.loc.start.offset,
+                endIndex: lastNodeFound.loc.end.offset,
+              });
+            }
+          }
+        }
+      },
+    };
+  },
+};