fix: do not report duplicate keys errors for keys added by merge keys

src/__tests__/fixtures/merge-keys-with-duplicate-props.yaml

@@ -0,0 +1,36 @@
+openapi: 3.0.0
+
+x-format-version: "1.0"
+
+info:
+  title: Merge key issue
+  description: https://yaml.org/type/merge.html
+  version: 1.0.0
+
+x-center: &CENTER
+  x: 1
+  y: 2
+
+x-left: &LEFT
+  x: 0
+  y: 2
+
+x-big: &BIG
+  r: 10
+
+x-small: &SMALL
+  r: 1
+
+x-one:
+  <<: *CENTER
+  r: 10
+  label: center/big
+x-two:
+  <<: [*CENTER, *BIG]
+  label: center/big
+x-three:
+  <<: [*BIG, *LEFT, *SMALL]
+  x: 1
+  label: center/big
+
+paths: {}

src/__tests__/parseWithPointers.spec.ts

@@ -7,6 +7,10 @@ import { HugeYAML } from './fixtures/huge-yaml';
 
 const diverse = fs.readFileSync(path.join(__dirname, './fixtures/diverse.yaml'), 'utf-8');
 const duplicateMergeKeys = fs.readFileSync(path.join(__dirname, './fixtures/duplicate-merge-keys.yaml'), 'utf-8');
+const mergeKeysWithDuplicateProperties = fs.readFileSync(
+  path.join(__dirname, './fixtures/merge-keys-with-duplicate-props.yaml'),
+  'utf-8'
+);
 const spectral481 = fs.readFileSync(path.join(__dirname, './fixtures/spectral-481.yaml'), 'utf-8');
 
 describe('yaml parser', () => {
@@ -428,6 +432,56 @@ european-cities: &cities
       });
     });
 
+    test('handles overrides #2', () => {
+      const result = parseWithPointers(mergeKeysWithDuplicateProperties, {
+        mergeKeys: true,
+        ignoreDuplicateKeys: false,
+      });
+
+      expect(result.data).toEqual({
+        openapi: '3.0.0',
+        'x-format-version': '1.0',
+        info: {
+          description: 'https://yaml.org/type/merge.html',
+          title: 'Merge key issue',
+          version: '1.0.0',
+        },
+        'x-center': {
+          x: 1,
+          y: 2,
+        },
+        'x-left': {
+          x: 0,
+          y: 2,
+        },
+        'x-big': {
+          r: 10,
+        },
+        'x-small': {
+          r: 1,
+        },
+        'x-one': {
+          x: 1,
+          y: 2,
+          r: 10,
+          label: 'center/big',
+        },
+        'x-two': {
+          x: 1,
+          y: 2,
+          r: 10,
+          label: 'center/big',
+        },
+        'x-three': {
+          x: 1,
+          y: 2,
+          r: 10,
+          label: 'center/big',
+        },
+        paths: {},
+      });
+    });
+
     test('handles duplicate merge keys', () => {
       const result = parseWithPointers(duplicateMergeKeys, { mergeKeys: true });
 
@@ -440,5 +494,23 @@ european-cities: &cities
         t: 4,
       });
     });
+
+    test('does not report duplicate merge keys', () => {
+      const result = parseWithPointers(duplicateMergeKeys, {
+        mergeKeys: true,
+        ignoreDuplicateKeys: false,
+      });
+
+      expect(result.diagnostics).toEqual([]);
+    });
+
+    test('does not report duplicate errors for merged keys', () => {
+      const result = parseWithPointers(mergeKeysWithDuplicateProperties, {
+        mergeKeys: true,
+        ignoreDuplicateKeys: false,
+      });
+
+      expect(result.diagnostics).toEqual([]);
+    });
   });
 });

src/parseWithPointers.ts

@@ -73,20 +73,29 @@ export const walkAST = (
       case Kind.MAP: {
         const container = {};
         // note, we don't handle null aka '~' keys on purpose
-        for (const mapping of (node as YamlMap).mappings) {
-          // typing is broken, value might be null
-          if (mapping.key.value in container) {
-            if (options !== void 0 && options.json === false) {
-              throw new Error('Duplicate YAML mapping key encountered');
-            }
+        const seenKeys: string[] = [];
+        const handleMergeKeys = options !== void 0 && options.mergeKeys === true;
+        const handleDuplicates = (options !== void 0 && options.json === false) || duplicatedMappingKeys !== void 0;
 
-            if (duplicatedMappingKeys !== void 0) {
-              duplicatedMappingKeys.push(mapping.key);
+        for (const mapping of (node as YamlMap).mappings) {
+          const key = mapping.key.value;
+
+          if (handleDuplicates && (!handleMergeKeys || key !== SpecialMappingKeys.MergeKey)) {
+            if (seenKeys.includes(mapping.key.value)) {
+              if (options !== void 0 && options.json === false) {
+                throw new Error('Duplicate YAML mapping key encountered');
+              }
+
+              if (duplicatedMappingKeys !== void 0) {
+                duplicatedMappingKeys.push(mapping.key);
+              }
+            } else {
+              seenKeys.push(key);
             }
           }
 
           // https://yaml.org/type/merge.html merge keys, not a part of YAML spec
-          if (options !== void 0 && options.mergeKeys === true && mapping.key.value === SpecialMappingKeys.MergeKey) {
+          if (handleMergeKeys && key === SpecialMappingKeys.MergeKey) {
             Object.assign(container, reduceMergeKeys(walkAST(mapping.value, options, duplicatedMappingKeys)));
           } else {
             container[mapping.key.value] = walkAST(mapping.value, options, duplicatedMappingKeys);