Dedupe duplicate properties (#5830)

* add `collapseDuplicateDeclarations`

This will allow us to remove duplicate declarations. This occurs when
you are using `@apply` for example.

The reason I implemented it as a separate step, is because this doesn't
only happen for `@apply`, but it also happens if you do something like:

```js
addComponents({ '.btn-blue, .btm-red': { padding: '10px' } })
```

So instead of tracking down every place this is happening, it now
happens at the very end.

* use new plugin in processTailwindFeatures

* add/update tests by removing duplicate declarations

* update changelog

src/lib/collapseDuplicateDeclarations.js

@@ -0,0 +1,28 @@
+export default function collapseDuplicateDeclarations() {
+  return (root) => {
+    root.walkRules((node) => {
+      let seen = new Map()
+      let droppable = new Set([])
+
+      node.walkDecls((decl) => {
+        // This could happen if we have nested selectors. In that case the
+        // parent will loop over all its declarations but also the declarations
+        // of nested rules. With this we ensure that we are shallowly checking
+        // declarations.
+        if (decl.parent !== node) {
+          return
+        }
+
+        if (seen.has(decl.prop)) {
+          droppable.add(seen.get(decl.prop))
+        }
+
+        seen.set(decl.prop, decl)
+      })
+
+      for (let decl of droppable) {
+        decl.remove()
+      }
+    })
+  }
+}

src/processTailwindFeatures.js

@@ -5,6 +5,7 @@ import evaluateTailwindFunctions from './lib/evaluateTailwindFunctions'
 import substituteScreenAtRules from './lib/substituteScreenAtRules'
 import resolveDefaultsAtRules from './lib/resolveDefaultsAtRules'
 import collapseAdjacentRules from './lib/collapseAdjacentRules'
+import collapseDuplicateDeclarations from './lib/collapseDuplicateDeclarations'
 import detectNesting from './lib/detectNesting'
 import { createContext } from './lib/setupContextUtils'
 import { issueFlagNotices } from './featureFlags'
@@ -42,5 +43,6 @@ export default function processTailwindFeatures(setupContext) {
     substituteScreenAtRules(context)(root, result)
     resolveDefaultsAtRules(context)(root, result)
     collapseAdjacentRules(context)(root, result)
+    collapseDuplicateDeclarations(context)(root, result)
   }
 }