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/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)
   }
 }