diff --git a/CHANGELOG.md b/CHANGELOG.md
index 070c8dba3d98..933fa5f9ab49 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Remove invalid `outline-hidden` utility ([#9147](https://github.com/tailwindlabs/tailwindcss/pull/9147))
 - Honor the `hidden` attribute on elements in preflight ([#9174](https://github.com/tailwindlabs/tailwindcss/pull/9174))
 - Don't stop watching atomically renamed files ([#9173](https://github.com/tailwindlabs/tailwindcss/pull/9173))
+- Re-use existing entries in the rule cache ([#9208](https://github.com/tailwindlabs/tailwindcss/pull/9208))
+- Don't output duplicate utilities ([#9208](https://github.com/tailwindlabs/tailwindcss/pull/9208))
 
 ## [3.1.8] - 2022-08-05
 
diff --git a/src/lib/expandTailwindAtRules.js b/src/lib/expandTailwindAtRules.js
index a0c1635a0fba..30b470800626 100644
--- a/src/lib/expandTailwindAtRules.js
+++ b/src/lib/expandTailwindAtRules.js
@@ -177,16 +177,12 @@ export default function expandTailwindAtRules(context) {
     let classCacheCount = context.classCache.size
 
     env.DEBUG && console.time('Generate rules')
-    let rules = generateRules(candidates, context)
+    generateRules(candidates, context)
     env.DEBUG && console.timeEnd('Generate rules')
 
     // We only ever add to the classCache, so if it didn't grow, there is nothing new.
     env.DEBUG && console.time('Build stylesheet')
     if (context.stylesheetCache === null || context.classCache.size !== classCacheCount) {
-      for (let rule of rules) {
-        context.ruleCache.add(rule)
-      }
-
       context.stylesheetCache = buildStylesheet([...context.ruleCache], context)
     }
     env.DEBUG && console.timeEnd('Build stylesheet')
diff --git a/src/lib/generateRules.js b/src/lib/generateRules.js
index 0f441fb18018..c99826e88971 100644
--- a/src/lib/generateRules.js
+++ b/src/lib/generateRules.js
@@ -649,8 +649,37 @@ function inKeyframes(rule) {
   return rule.parent && rule.parent.type === 'atrule' && rule.parent.name === 'keyframes'
 }
 
+function getImportantStrategy(important) {
+  if (important === true) {
+    return (rule) => {
+      if (inKeyframes(rule)) {
+        return
+      }
+
+      rule.walkDecls((d) => {
+        if (d.parent.type === 'rule' && !inKeyframes(d.parent)) {
+          d.important = true
+        }
+      })
+    }
+  }
+
+  if (typeof important === 'string') {
+    return (rule) => {
+      if (inKeyframes(rule)) {
+        return
+      }
+
+      rule.selectors = rule.selectors.map((selector) => {
+        return `${important} ${selector}`
+      })
+    }
+  }
+}
+
 function generateRules(candidates, context) {
   let allRules = []
+  let strategy = getImportantStrategy(context.tailwindConfig.important)
 
   for (let candidate of candidates) {
     if (context.notClassCache.has(candidate)) {
@@ -658,7 +687,11 @@ function generateRules(candidates, context) {
     }
 
     if (context.classCache.has(candidate)) {
-      allRules.push(context.classCache.get(candidate))
+      continue
+    }
+
+    if (context.candidateRuleCache.has(candidate)) {
+      allRules = allRules.concat(Array.from(context.candidateRuleCache.get(candidate)))
       continue
     }
 
@@ -670,47 +703,27 @@ function generateRules(candidates, context) {
     }
 
     context.classCache.set(candidate, matches)
-    allRules.push(matches)
-  }
 
-  // Strategy based on `tailwindConfig.important`
-  let strategy = ((important) => {
-    if (important === true) {
-      return (rule) => {
-        rule.walkDecls((d) => {
-          if (d.parent.type === 'rule' && !inKeyframes(d.parent)) {
-            d.important = true
-          }
-        })
-      }
-    }
+    let rules = context.candidateRuleCache.get(candidate) ?? new Set()
+    context.candidateRuleCache.set(candidate, rules)
 
-    if (typeof important === 'string') {
-      return (rule) => {
-        rule.selectors = rule.selectors.map((selector) => {
-          return `${important} ${selector}`
-        })
-      }
-    }
-  })(context.tailwindConfig.important)
+    for (const match of matches) {
+      let [{ sort, layer, options }, rule] = match
 
-  return allRules.flat(1).map(([{ sort, layer, options }, rule]) => {
-    if (options.respectImportant) {
-      if (strategy) {
+      if (options.respectImportant && strategy) {
         let container = postcss.root({ nodes: [rule.clone()] })
-        container.walkRules((r) => {
-          if (inKeyframes(r)) {
-            return
-          }
-
-          strategy(r)
-        })
+        container.walkRules(strategy)
         rule = container.nodes[0]
       }
+
+      let newEntry = [sort | context.layerOrder[layer], rule]
+      rules.add(newEntry)
+      context.ruleCache.add(newEntry)
+      allRules.push(newEntry)
     }
+  }
 
-    return [sort | context.layerOrder[layer], rule]
-  })
+  return allRules
 }
 
 function isArbitraryValue(input) {
diff --git a/src/lib/setupContextUtils.js b/src/lib/setupContextUtils.js
index b0552b05d50c..1fd72e301c4e 100644
--- a/src/lib/setupContextUtils.js
+++ b/src/lib/setupContextUtils.js
@@ -873,6 +873,7 @@ export function createContext(tailwindConfig, changedContent = [], root = postcs
   let context = {
     disposables: [],
     ruleCache: new Set(),
+    candidateRuleCache: new Map(),
     classCache: new Map(),
     applyClassCache: new Map(),
     notClassCache: new Set(),
diff --git a/tests/important-boolean.test.js b/tests/important-boolean.test.js
index 20fc4d1ee598..48a7c10f89f8 100644
--- a/tests/important-boolean.test.js
+++ b/tests/important-boolean.test.js
@@ -1,7 +1,8 @@
 import fs from 'fs'
 import path from 'path'
+import * as sharedState from '../src/lib/sharedState'
 
-import { run, css } from './util/run'
+import { run, css, html } from './util/run'
 
 test('important boolean', () => {
   let config = {
@@ -63,3 +64,74 @@ test('important boolean', () => {
     expect(result.css).toMatchFormattedCss(expected)
   })
 })
+
+// This is in a describe block so we can use `afterEach` :)
+describe('duplicate elision', () => {
+  let filePath = path.resolve(__dirname, './important-boolean-duplicates.test.html')
+
+  afterEach(async () => await fs.promises.unlink(filePath))
+
+  test('important rules are not duplicated when rebuilding', async () => {
+    let config = {
+      important: true,
+      content: [filePath],
+    }
+
+    await fs.promises.writeFile(
+      config.content[0],
+      html`
+        <div class="ml-2"></div>
+        <div class="ml-4"></div>
+      `
+    )
+
+    let input = css`
+      @tailwind utilities;
+    `
+
+    let result = await run(input, config)
+    let allContexts = Array.from(sharedState.contextMap.values())
+
+    let context = allContexts[allContexts.length - 1]
+
+    let ruleCacheSize1 = context.ruleCache.size
+
+    expect(result.css).toMatchFormattedCss(css`
+      .ml-2 {
+        margin-left: 0.5rem !important;
+      }
+      .ml-4 {
+        margin-left: 1rem !important;
+      }
+    `)
+
+    await fs.promises.writeFile(
+      config.content[0],
+      html`
+        <div class="ml-2"></div>
+        <div class="ml-6"></div>
+      `
+    )
+
+    result = await run(input, config)
+
+    let ruleCacheSize2 = context.ruleCache.size
+
+    expect(result.css).toMatchFormattedCss(css`
+      .ml-2 {
+        margin-left: 0.5rem !important;
+      }
+      .ml-4 {
+        margin-left: 1rem !important;
+      }
+      .ml-6 {
+        margin-left: 1.5rem !important;
+      }
+    `)
+
+    // The rule cache was effectively doubling in size previously
+    // because the rule cache was never de-duped
+    // This ensures this behavior doesn't return
+    expect(ruleCacheSize2 - ruleCacheSize1).toBeLessThan(10)
+  })
+})