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node_modules/stylelint/lib/rules/no-duplicate-selectors/README.md

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+# no-duplicate-selectors
+
+Disallow duplicate selectors within a stylesheet.
+
+```css
+    .foo {} .bar {} .foo {}
+/** ↑              ↑
+ * These duplicates */
+```
+
+This rule checks for two types of duplication:
+
+-   Duplication of a single selector with a rule's selector list, e.g. `a, b, a {}`.
+-   Duplication of a selector list within a stylesheet, e.g. `a, b {} a, b {}`. Duplicates are found even if the selectors come in different orders or have different spacing, e.g. `a d, b > c {} b>c, a   d {}`.
+
+The same selector *is* allowed to repeat in the following circumstances:
+
+-   It is used in different selector lists, e.g. `a {} a, b {}`.
+-   The duplicates are determined to originate in different stylesheets, e.g. you have concatenated or compiled files in a way that produces sourcemaps for PostCSS to read, e.g. postcss-import).
+-   The duplicates are in rules with different parent nodes, e.g. inside and outside of a media query.
+
+This rule resolves nested selectors. So `a b {} a { & b {} }` counts as a warning, because the resolved selectors end up with a duplicate.
+
+## Options
+
+### `true`
+
+The following patterns are considered warnings:
+
+```css
+.foo,
+.bar,
+.foo {}
+```
+
+```css
+.foo {}
+.bar {}
+.foo {}
+```
+
+```css
+.foo .bar {}
+.bar {}
+.foo .bar {}
+```
+
+```css
+@media (min-width: 10px) {
+  .foo {}
+  .foo {}
+}
+```
+
+```css
+.foo, .bar {}
+.bar, .foo {}
+```
+
+```css
+a .foo, b + .bar {}
+b+.bar,
+a
+  .foo {}
+```
+
+```css
+a b {}
+a {
+  & b {}
+}
+```
+
+The following patterns are *not* considered warnings:
+
+```css
+.foo {}
+@media (min-width: 10px) {
+  .foo {}
+}
+```
+
+```css
+.foo {
+  .foo {}
+}
+```
+
+```css
+.foo {}
+.bar {}
+.foo .bar {}
+.bar .foo {}
+```
+
+```css
+a b {}
+a {
+  & b,
+  & c {}
+}
+```

node_modules/stylelint/lib/rules/no-duplicate-selectors/index.js

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+"use strict"
+
+const findAtRuleContext = require("../../utils/findAtRuleContext")
+const isKeyframeRule = require("../../utils/isKeyframeRule")
+const nodeContextLookup = require("../../utils/nodeContextLookup")
+const report = require("../../utils/report")
+const ruleMessages = require("../../utils/ruleMessages")
+const validateOptions = require("../../utils/validateOptions")
+const _ = require("lodash")
+const normalizeSelector = require("normalize-selector")
+const resolvedNestedSelector = require("postcss-resolve-nested-selector")
+
+const ruleName = "no-duplicate-selectors"
+
+const messages = ruleMessages(ruleName, {
+  rejected: selector => `Unexpected duplicate selector "${selector}"`,
+})
+
+const rule = function (actual) {
+  return (root, result) => {
+    const validOptions = validateOptions(result, ruleName, { actual })
+    if (!validOptions) {
+      return
+    }
+
+    // The top level of this map will be rule sources.
+    // Each source maps to another map, which maps rule parents to a set of selectors.
+    // This ensures that selectors are only checked against selectors
+    // from other rules that share the same parent and the same source.
+    const selectorContextLookup = nodeContextLookup()
+
+    root.walkRules(rule => {
+      if (isKeyframeRule(rule)) {
+        return
+      }
+
+      const contextSelectorSet = selectorContextLookup.getContext(rule, findAtRuleContext(rule))
+      const resolvedSelectors = rule.selectors.reduce((result, selector) => {
+        return _.union(result, resolvedNestedSelector(selector, rule))
+      }, [])
+      const normalizedSelectorList = resolvedSelectors.map(normalizeSelector)
+
+      // Complain if the same selector list occurs twice
+
+      // Sort the selectors list so that the order of the constituents
+      // doesn't matter
+      const sortedSelectorList = normalizedSelectorList.slice().sort().join(",")
+      if (contextSelectorSet.has(sortedSelectorList)) {
+        // If the selector isn't nested we can use its raw value; otherwise,
+        // we have to approximate something for the message -- which is close enough
+        const isNestedSelector = resolvedSelectors.join(",") !== rule.selectors.join(",")
+        const selectorForMessage = isNestedSelector ? resolvedSelectors.join(", ") : rule.selector
+        return report({
+          result,
+          ruleName,
+          node: rule,
+          message: messages.rejected(selectorForMessage),
+        })
+      }
+
+      // We're treating the Map created by nodeContextLookup as a Set
+      contextSelectorSet.set(sortedSelectorList, null)
+
+      // Or complain if one selector list contains the same selector more than one
+      rule.selectors.forEach((selector, i) => {
+        if (_.includes(normalizedSelectorList.slice(0, i), normalizeSelector(selector))) {
+          report({
+            result,
+            ruleName,
+            node: rule,
+            message: messages.rejected(selector),
+          })
+        }
+      })
+    })
+  }
+}
+
+rule.ruleName = ruleName
+rule.messages = messages
+module.exports = rule