fix(core): can't have multiple CfnRules in a Stack (#9500)

When we changed the merging behavior in #8251,
we forgot to account for the 'Rules' section.
To prevent that error from happening again,
let's default to merging objects without duplicates.

Fixes #9485

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/core/lib/stack.ts

@@ -966,16 +966,8 @@ function mergeSection(section: string, val1: any, val2: any): any {
       return val1 ?? val2;
     case 'Transform':
       return mergeSets(val1, val2);
-    case 'Resources':
-    case 'Conditions':
-    case 'Parameters':
-    case 'Outputs':
-    case 'Mappings':
-    case 'Metadata':
-      return mergeObjectsWithoutDuplicates(section, val1, val2);
     default:
-      throw new Error(`CDK doesn't know how to merge two instances of the CFN template section '${section}' - ` +
-        'please remove one of them from your code');
+      return mergeObjectsWithoutDuplicates(section, val1, val2);
   }
 }
 

packages/@aws-cdk/core/test/test.rule.ts

@@ -29,4 +29,20 @@ export = {
 
     test.done();
   },
+
+  'a template can contain multiple Rules'(test: Test) {
+    const stack = new Stack();
+
+    new CfnRule(stack, 'Rule1');
+    new CfnRule(stack, 'Rule2');
+
+    test.deepEqual(toCloudFormation(stack), {
+      Rules: {
+        Rule1: {},
+        Rule2: {},
+      },
+    });
+
+    test.done();
+  },
 };