【Weabpack v1 → v3】解決「Conflict: Multiple assets emit to the same filename bundle.js」

下のようになっている箇所を

output: {
path: __dirname + "/dist",
filename: "bundle.js",
},

このようにする

output: {
path: __dirname + "/dist",
filename: "[name].js",
},

First of all, on your output, you are specifying the filename to app.js which makes sense for me that the output will still be app.js. If you want to make it dynamic, then just use "filename": "[name].js".