diff --git a/CHANGELOG.md b/CHANGELOG.md
index 95603dd65a98..eda8c5b631fc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure we can use `<` and `>` characters in modifiers ([#6851](https://github.com/tailwindlabs/tailwindcss/pull/6851))
 - Validate `theme()` works in arbitrary values ([#6852](https://github.com/tailwindlabs/tailwindcss/pull/6852))
 - Properly detect `theme()` value usage in arbitrary properties ([#6854](https://github.com/tailwindlabs/tailwindcss/pull/6854))
+- Improve collapsing of duplicate declarations ([#6856](https://github.com/tailwindlabs/tailwindcss/pull/6856))
 
 ## [3.0.8] - 2021-12-28
 
diff --git a/src/lib/collapseDuplicateDeclarations.js b/src/lib/collapseDuplicateDeclarations.js
index 9164f822ea78..d310d58b88b6 100644
--- a/src/lib/collapseDuplicateDeclarations.js
+++ b/src/lib/collapseDuplicateDeclarations.js
@@ -3,6 +3,7 @@ export default function collapseDuplicateDeclarations() {
     root.walkRules((node) => {
       let seen = new Map()
       let droppable = new Set([])
+      let byProperty = new Map()
 
       node.walkDecls((decl) => {
         // This could happen if we have nested selectors. In that case the
@@ -14,15 +15,79 @@ export default function collapseDuplicateDeclarations() {
         }
 
         if (seen.has(decl.prop)) {
-          droppable.add(seen.get(decl.prop))
+          // Exact same value as what we have seen so far
+          if (seen.get(decl.prop).value === decl.value) {
+            // Keep the last one, drop the one we've seen so far
+            droppable.add(seen.get(decl.prop))
+            // Override the existing one with the new value. This is necessary
+            // so that if we happen to have more than one declaration with the
+            // same value, that we keep removing the previous one. Otherwise we
+            // will only remove the *first* one.
+            seen.set(decl.prop, decl)
+            return
+          }
+
+          // Not the same value, so we need to check if we can merge it so
+          // let's collect it first.
+          if (!byProperty.has(decl.prop)) {
+            byProperty.set(decl.prop, new Set())
+          }
+
+          byProperty.get(decl.prop).add(seen.get(decl.prop))
+          byProperty.get(decl.prop).add(decl)
         }
 
         seen.set(decl.prop, decl)
       })
 
+      // Drop all the duplicate declarations with the exact same value we've
+      // already seen so far.
       for (let decl of droppable) {
         decl.remove()
       }
+
+      // Analyze the declarations based on its unit, drop all the declarations
+      // with the same unit but the last one in the list.
+      for (let declarations of byProperty.values()) {
+        let byUnit = new Map()
+
+        for (let decl of declarations) {
+          let unit = resolveUnit(decl.value)
+          if (unit === null) {
+            // We don't have a unit, so should never try and collapse this
+            // value. This is because we can't know how to do it in a correct
+            // way (e.g.: overrides for older browsers)
+            continue
+          }
+
+          if (!byUnit.has(unit)) {
+            byUnit.set(unit, new Set())
+          }
+
+          byUnit.get(unit).add(decl)
+        }
+
+        for (let declarations of byUnit.values()) {
+          // Get all but the last one
+          let removableDeclarations = Array.from(declarations).slice(0, -1)
+
+          for (let decl of removableDeclarations) {
+            decl.remove()
+          }
+        }
+      }
     })
   }
 }
+
+let UNITLESS_NUMBER = Symbol('unitless-number')
+
+function resolveUnit(input) {
+  let result = /^-?\d*.?\d+([\w%]+)?$/g.exec(input)
+
+  if (result) {
+    return result[1] ?? UNITLESS_NUMBER
+  }
+
+  return null
+}
diff --git a/tests/apply.test.js b/tests/apply.test.js
index c74bb2875f5d..59807a633129 100644
--- a/tests/apply.test.js
+++ b/tests/apply.test.js
@@ -351,6 +351,7 @@ test('@applying classes from outside a @layer respects the source order', async
   await run(input, config).then((result) => {
     return expect(result.css).toMatchFormattedCss(css`
       .baz {
+        text-decoration-line: underline;
         text-decoration-line: none;
       }
 
diff --git a/tests/collapse-duplicate-declarations.test.js b/tests/collapse-duplicate-declarations.test.js
new file mode 100644
index 000000000000..db165186469b
--- /dev/null
+++ b/tests/collapse-duplicate-declarations.test.js
@@ -0,0 +1,175 @@
+import { run, css, html } from './util/run'
+
+it('should collapse duplicate declarations with the same units (px)', () => {
+  let config = {
+    content: [{ raw: html`<div class="example"></div>` }],
+    corePlugins: { preflight: false },
+    plugins: [],
+  }
+
+  let input = css`
+    @tailwind utilities;
+
+    @layer utilities {
+      .example {
+        height: 100px;
+        height: 200px;
+      }
+    }
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .example {
+        height: 200px;
+      }
+    `)
+  })
+})
+
+it('should collapse duplicate declarations with the same units (no unit)', () => {
+  let config = {
+    content: [{ raw: html`<div class="example"></div>` }],
+    corePlugins: { preflight: false },
+    plugins: [],
+  }
+
+  let input = css`
+    @tailwind utilities;
+
+    @layer utilities {
+      .example {
+        line-height: 3;
+        line-height: 2;
+      }
+    }
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .example {
+        line-height: 2;
+      }
+    `)
+  })
+})
+
+it('should not collapse duplicate declarations with the different units', () => {
+  let config = {
+    content: [{ raw: html`<div class="example"></div>` }],
+    corePlugins: { preflight: false },
+    plugins: [],
+  }
+
+  let input = css`
+    @tailwind utilities;
+
+    @layer utilities {
+      .example {
+        height: 100px;
+        height: 50%;
+      }
+    }
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .example {
+        height: 100px;
+        height: 50%;
+      }
+    `)
+  })
+})
+
+it('should collapse the duplicate declarations with the same unit, but leave the ones with different units', () => {
+  let config = {
+    content: [{ raw: html`<div class="example"></div>` }],
+    corePlugins: { preflight: false },
+    plugins: [],
+  }
+
+  let input = css`
+    @tailwind utilities;
+
+    @layer utilities {
+      .example {
+        height: 100px;
+        height: 50%;
+        height: 20vh;
+        height: 200px;
+        height: 100%;
+        height: 30vh;
+      }
+    }
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .example {
+        height: 200px;
+        height: 100%;
+        height: 30vh;
+      }
+    `)
+  })
+})
+
+it('should collapse the duplicate declarations with the exact same value', () => {
+  let config = {
+    content: [{ raw: html`<div class="example"></div>` }],
+    corePlugins: { preflight: false },
+    plugins: [],
+  }
+
+  let input = css`
+    @tailwind utilities;
+
+    @layer utilities {
+      .example {
+        height: var(--value);
+        color: blue;
+        height: var(--value);
+      }
+    }
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .example {
+        color: blue;
+        height: var(--value);
+      }
+    `)
+  })
+})
+
+it('should work on a real world example', () => {
+  let config = {
+    content: [{ raw: html`<div class="h-available"></div>` }],
+    corePlugins: { preflight: false },
+    plugins: [],
+  }
+
+  let input = css`
+    @tailwind utilities;
+
+    @layer utilities {
+      .h-available {
+        height: 100%;
+        height: 100vh;
+        height: -webkit-fill-available;
+      }
+    }
+  `
+
+  return run(input, config).then((result) => {
+    expect(result.css).toMatchFormattedCss(css`
+      .h-available {
+        height: 100%;
+        height: 100vh;
+        height: -webkit-fill-available;
+      }
+    `)
+  })
+})