fix: show warning when hot is enabled with HMR plugin in config

Author: snitin315

lib/Server.js

@@ -1056,16 +1056,20 @@ class Server {
       // TODO remove after drop webpack v4 support
       compiler.options.plugins = compiler.options.plugins || [];
 
-      if (
-        this.options.hot &&
-        !compiler.options.plugins.find(
+      if (this.options.hot) {
+        const HMRPluginExists = compiler.options.plugins.find(
           (p) => p.constructor === webpack.HotModuleReplacementPlugin
-        )
-      ) {
-        // apply the HMR plugin, if it didn't exist before.
-        const plugin = new webpack.HotModuleReplacementPlugin();
+        );
 
-        plugin.apply(compiler);
+        if (HMRPluginExists) {
+          this.logger.warn(
+            `"hot: true" automatically applies HMR plugin, you don't have to add it manually to your webpack configuration.`
+          );
+        } else {
+          // apply the HMR plugin
+          const plugin = new webpack.HotModuleReplacementPlugin();
+          plugin.apply(compiler);
+        }
       }
     });