use a Set to make files unique

During debugging I noticed that some of the files were in the list
multiple times, using a Set solves that issues. While this is a little
band-aid it helps for now. Following commits will include a more proper
fix.

packages/@tailwindcss-postcss/src/index.ts

@@ -104,11 +104,13 @@ function tailwindcss(opts: PluginOptions = {}): AcceptedPlugin {
               })
             }
 
-            let files = result.messages.flatMap((message) => {
-              if (message.type !== 'dependency') return []
-              return message.file
-            })
-            files.push(inputFile)
+            let files = new Set(
+              result.messages.flatMap((message) => {
+                if (message.type !== 'dependency') return []
+                return message.file
+              }),
+            )
+            files.add(inputFile)
 
             for (let file of files) {
               let changedTime = fs.statSync(file, { throwIfNoEntry: false })?.mtimeMs ?? null